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.rs

@@ -1395,7 +1395,7 @@ mod tests {
 
             // dedent "else" on the line after a closing paren
             append(&mut buffer, "\n  else:\n", cx);
-            assert_eq!(buffer.text(), "if a:\n  b(\n  )\nelse:\n");
+            assert_eq!(buffer.text(), "if a:\n  b(\n  )\nelse:\n  ");
 
             buffer
         });