perf: native io function instead of File class

[neo451]

• Use native lua io function to replace File class that depends on plenary

faster after benchmark:

local function bench(f, name)
  local t = os.clock()

  for _ = 1, 100000 do
    f()
  end

  print(name, os.clock() - t)
end

local note = require "obsidian.note"
local async = require "plenary.async"

bench(function()
  note.from_file_async2 "~/Notes/21-30 Tinker/12 Open Source/12.1 obsidian.nvim/lsp proposal.md"
end, "new")

bench(function()
  async.util.block_on(function()
    note.from_file_async "~/Notes/21-30 Tinker/12 Open Source/12.1 obsidian.nvim/lsp proposal.md"
  end)
end, "old")

-- new 5s
-- old 6s

• improved some tests

PR Checklist

• The PR contains a description of the changes
• I read the [CONTRIBUTING.md] file
• The CHANGELOG.md is updated
• The changes are documented in the README.md file
• The code complies with make chores (for style, lint, types, and tests)