feat!: Use a more commonly supported characters for fat headline

 ## What

This commit replaces the character `🬂`
([Block Sextant-12 (U+1FB02)](https://unicodeplus.com/U+1FB02))
with `▀`
([Upper Half Block (U+2580)](https://unicodeplus.com/U+2580))
to be used for the `fat_headline_lower_string` configuration everywhere.

 ## Why

The _Block Sextant-12_ character is part of a Unicode block named
[_Symbols for Legacy Computing_](https://en.wikipedia.org/wiki/Symbols_for_Legacy_Computing).

This character seems to not be supported by a lot of fonts and many
terminals, as well as other applications (one might even see a "missing
character" square while browsing this PR and links I provided).

I have for example confirmed that it would not work with the following:
- Terminal apps:
  - macOS' Terminal.app
  - iTerm2
  - Alacritty with `builtin_box_drawing` set to `false`
- Fonts:
  - Monaco
  - MonaspiceNe Nerd Font
  - SFMono Nerd Font

_Upper Half Block_ is slightly thicker than _Block Sextant-12_, but it
also appears to be more widely supported accross terminals and fonts.

I think a default configuration that will work in most environments
would provide a better user experience, and would likely prevent many
developers from spending hours diving into the unknown territories of
font rendering, Unicode blocks, NeoVim LUA configuration, etc.

 ## How

A simple script that replaces all instances of the _Block Sextant-12_
character in the repository:

```sh
 ag -l \U0001fb02 | xargs -n1 sed -i'' -e "s/\U0001fb02/▀/"
```

 ## Anything else?

 ### Alternative

An alternative to _Upper Half Block_ would be the thinner `▔`
([Upper One Eighth Block (U+2594)](https://unicodeplus.com/U+2594)).

 ### Matching "lower" and "upper"

If it is important for the width of the fat headline top and bottom
padding be identical, then we could also change
`fat_headline_upper_string` to use a character of the same width.
Respectively, in order of appearance above:
- `▄` ([Lower Half Block (U+2584)](https://unicodeplus.com/U+2584))
- `▁` ([Lower One Eighth Block (U+2581)](https://unicodeplus.com/U+2581))

Author: davidstosik

README.md

@@ -114,8 +114,8 @@ require("headlines").setup {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     rmd = {
         query = vim.treesitter.parse_query(
@@ -157,8 +157,8 @@ require("headlines").setup {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     norg = {
         query = vim.treesitter.parse_query(
@@ -206,8 +206,8 @@ require("headlines").setup {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     org = {
         query = vim.treesitter.parse_query(
@@ -248,8 +248,8 @@ require("headlines").setup {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
 }
 ```

doc/headlines.txt

@@ -133,8 +133,8 @@ Default config: >
           quote_highlight = "Quote",
           quote_string = "┃",
           fat_headlines = true,
-          fat_headline_upper_string = "▃",
-          fat_headline_lower_string = "🬂",
+          fat_headline_upper_string = "▄",
+          fat_headline_lower_string = "▀",
       },
       rmd = {
           query = vim.treesitter.parse_query(
@@ -176,8 +176,8 @@ Default config: >
           quote_highlight = "Quote",
           quote_string = "┃",
           fat_headlines = true,
-          fat_headline_upper_string = "▃",
-          fat_headline_lower_string = "🬂",
+          fat_headline_upper_string = "▄",
+          fat_headline_lower_string = "▀",
       },
       norg = {
           query = vim.treesitter.parse_query(
@@ -225,8 +225,8 @@ Default config: >
           quote_highlight = "Quote",
           quote_string = "┃",
           fat_headlines = true,
-          fat_headline_upper_string = "▃",
-          fat_headline_lower_string = "🬂",
+          fat_headline_upper_string = "▄",
+          fat_headline_lower_string = "▀",
       },
       org = {
           query = vim.treesitter.parse_query(
@@ -267,8 +267,8 @@ Default config: >
           quote_highlight = "Quote",
           quote_string = "┃",
           fat_headlines = true,
-          fat_headline_upper_string = "▃",
-          fat_headline_lower_string = "🬂",
+          fat_headline_upper_string = "▄",
+          fat_headline_lower_string = "▀",
       },
   }
 

lua/headlines/init.lua

@@ -56,8 +56,8 @@ M.config = {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     rmd = {
         query = parse_query_save(
@@ -99,8 +99,8 @@ M.config = {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     norg = {
         query = parse_query_save(
@@ -148,8 +148,8 @@ M.config = {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
     org = {
         query = parse_query_save(
@@ -190,8 +190,8 @@ M.config = {
         quote_highlight = "Quote",
         quote_string = "┃",
         fat_headlines = true,
-        fat_headline_upper_string = "▃",
-        fat_headline_lower_string = "🬂",
+        fat_headline_upper_string = "▄",
+        fat_headline_lower_string = "▀",
     },
 }