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
         });

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"] }
+]

crates/languages/src/python/indents.scm

@@ -1,72 +1,17 @@
-(_ "(" ")" @end) @indent
 (_ "[" "]" @end) @indent
 (_ "{" "}" @end) @indent
+(_ "(" ")" @end) @indent
 
-(function_definition
-  ":" @start
-  body: (block) @indent
-)
-
-(if_statement
-  ":" @start
-  consequence: (block) @indent
-  alternative: (_)? @outdent
-)
-
-(else_clause
-  ":" @start
-  body: (block) @indent
-)
-
-(elif_clause
-  ":" @start
-  consequence: (block) @indent
-)
-
-(for_statement
-  ":" @start
-  body: (block) @indent
-)
-
-(with_statement
-  ":" @start
-  body: (block) @indent
-)
-
-(while_statement
-  ":" @start
-  body: (block) @indent
-)
-
-(match_statement
-  ":" @start
-  body: (block) @indent
-)
-
-(class_definition
-  ":" @start
-  body: (block) @indent
-)
-
-(case_clause
-  ":" @start
-  consequence: (block) @indent
-)
-
-(try_statement
-  ":" @start
-  body: (block) @indent
-  (except_clause)? @outdent
-  (else_clause)? @outdent
-  (finally_clause)? @outdent
-)
-
-(except_clause
-  ":" @start
-  (block) @indent
-)
-
-(finally_clause
-  ":" @start
-  (block) @indent
-)
+(function_definition) @start.def
+(class_definition) @start.class
+(if_statement) @start.if
+(for_statement) @start.for
+(while_statement) @start.while
+(with_statement) @start.with
+(match_statement) @start.match
+(try_statement) @start.try
+(elif_clause) @start.elif
+(else_clause) @start.else
+(except_clause) @start.except
+(finally_clause) @start.finally
+(case_pattern) @start.case