language: Add context-aware decrease indent for Python (#33370)

Closes #33238, follow-up to
https://github.com/zed-industries/zed/pull/29625.

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:

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

but,

```py
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.

crates/languages/src/python/config.toml

@@ -28,6 +28,11 @@ brackets = [
 
 auto_indent_using_last_non_empty_line = false
 debuggers = ["Debugpy"]
-significant_indentation = true
-increase_indent_pattern = "^\\s*(try)\\b.*:"
-decrease_indent_pattern = "^\\s*(else|elif|except|finally)\\b.*:"
+increase_indent_pattern = "^[^#].*:\\s*$"
+decrease_indent_patterns = [
+  { pattern = "^\\s*elif\\b.*:",    valid_after = ["if", "elif"] },
+  { pattern = "^\\s*else\\b.*:",    valid_after = ["if", "elif", "for", "while", "except"] },
+  { pattern = "^\\s*except\\b.*:",  valid_after = ["try", "except"] },
+  { pattern = "^\\s*finally\\b.*:", valid_after = ["try", "except", "else"] },
+  { pattern = "^\\s*case\\b.*:",    valid_after = ["match", "case"] }
+]