Add a few more rust highlighting patterns

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/languages/rust/highlights.scm

@@ -1,4 +1,5 @@
 (type_identifier) @type
+(primitive_type) @type.builtin
 
 (field_identifier) @property
 
@@ -11,37 +12,50 @@
       field: (field_identifier) @function.method)
   ])
 
-(function_item
-  name: (identifier) @function.definition)
+(function_item name: (identifier) @function.definition)
+(function_signature_item name: (identifier) @function.definition)
 
 [
   "async"
   "break"
   "const"
   "continue"
+  "default"
   "dyn"
   "else"
   "enum"
+  "extern"
   "for"
   "fn"
   "if"
+  "in"
   "impl"
   "let"
   "loop"
+  "macro_rules!"
   "match"
   "mod"
   "move"
   "pub"
   "return"
+  "static"
   "struct"
   "trait"
   "type"
   "use"
   "where"
   "while"
+  "union"
+  "unsafe"
+  (mutable_specifier)
+  (super)
 ] @keyword
 
-(string_literal) @string
+[
+  (string_literal)
+  (raw_string_literal)
+  (char_literal)
+] @string
 
 [
   (line_comment)