python: Improve docstring highlighting (#31628)

This PR broadens the highlighting for docstrings in Python. 

Previously, only the first docstring for e.g. type aliases was
highlighted in Python files. This happened as only the first occurrence
in the module was considered a docstring. With this change, now all
existing docstrings are actually highlighted as such.

| `main` | This PR | 
| --- | --- |
|
![main](https://github.com/user-attachments/assets/facc96a9-4e98-4063-8b93-d6e9884221ff)
|
![PR](https://github.com/user-attachments/assets/9da557a1-b327-466a-be87-65d6a811e24c)
|

Release Notes:

- Added more docstring highlights for Python.

crates/languages/src/python/highlights.scm

@@ -151,6 +151,12 @@
   "}" @punctuation.special) @embedded
 
 ; Docstrings.
+([
+  (expression_statement (assignment))
+  (type_alias_statement)
+]
+. (expression_statement (string) @string.doc)+)
+
 (module
   .(expression_statement (string) @string.doc)+)
 
@@ -173,13 +179,6 @@
   . (comment) @comment*
   . (expression_statement (string) @string.doc)+)
 
-(module
-  [
-    (expression_statement (assignment))
-    (type_alias_statement)
-  ]
-  . (expression_statement (string) @string.doc)+)
-
 (class_definition
   body: (block
     (expression_statement (assignment))