rust: Add static items to the outline (#15225)

Fixes #15208
Release Notes:

- Outline panel for Rust files now includes static items.

crates/languages/src/rust/outline.scm

@@ -59,6 +59,11 @@
     "const" @context
     name: (_) @name) @item
 
+(static_item
+    (visibility_modifier)? @context
+    "static" @context
+    name: (_) @name) @item
+
 (field_declaration
     (visibility_modifier)? @context
     name: (_) @name) @item