language: Add context-aware decrease indent for Python

[smitbarmase]

Closes #33238, follow-up to #29625. Blocked on tree-sitter/tree-sitter-python#304

Changes:

• Removed significant_indentation, which was the way to introduce indentation scoping in languages like Python. However, it turned out to be unnecessarily complicated to define and maintain.
• Introduced decrease_indent_patterns, which takes a pattern keyword to automatically outdent and valid_after keywords to treat as valid code points to snap to. The outdent happens to the most recent valid_after keyword that also has less or equal indentation than the currently typed keyword.

Fixes:

1. In Python, typing except, finally, else, and so on now automatically indents intelligently based on the context in which it appears. For instance:

try:
    if a == 1:
        try:
             b = 2
             ^  # <-- typing "except:" here would indent it to inner try block

but,

try:
    if a == 1:
        try:
             b = 2
    ^  # <-- typing "except:" here would indent it to outer try block

2. Fixes comments not maintaining indent.

Release Notes:

• Improved auto outdent for Python while typing keywords like except, else, finally, etc.
• Fixed the issue where comments in Python would not maintain their indentation.