Start on adding support for editing via the assistant panel (#14795)

Note that this shouldn't have any visible user-facing behavior yet. The
feature is incomplete but we wanna merge early to avoid a long-running
branch.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/language/src/buffer_tests.rs

@@ -2615,7 +2615,8 @@ fn rust_lang() -> Language {
             "impl" @context
             trait: (_)? @name
             "for"? @context
-            type: (_) @name) @item
+            type: (_) @name
+            body: (_ "{" (_)* "}")) @item
         (function_item
             "fn" @context
             name: (_) @name) @item