Introduce storage_class highlight scope

Inspired by VIM's StorageClass hi group

book/src/themes.md

@@ -151,6 +151,7 @@ We use a similar set of scopes as
   - `operator` - `or`, `in`
   - `directive` - Preprocessor directives (`#if` in C) 
   - `function` - `fn`, `func`
+  - `storage_class` - `static`, `mut`, `const`, `ref`
 
 - `operator` - `||`, `+=`, `>`
 

runtime/queries/c/highlights.scm

@@ -1,8 +1,9 @@
+(storage_class_specifier) @keyword.storage_class
+
 "goto" @keyword
 "register" @keyword
 "break" @keyword
 "case" @keyword
-"const" @keyword
 "continue" @keyword
 "default" @keyword
 "do" @keyword
@@ -14,7 +15,6 @@
 "inline" @keyword
 "return" @keyword
 "sizeof" @keyword
-"static" @keyword
 "struct" @keyword
 "switch" @keyword
 "typedef" @keyword

runtime/queries/rust/highlights.scm

@@ -156,16 +156,19 @@
   "macro_rules!"
 
   "let"
-  "ref"
-  "move"
 
-  "dyn"
-  "static"
-  "const"
   "async"
 ] @keyword
 
-(mutable_specifier) @keyword.mut
+[
+  ("&" (identifier))
+  (mutable_specifier)
+  "static"
+  "const"
+  "ref"
+  "move"
+  "dyn"
+] @keyword.storage_class
 
 ; TODO: variable.mut to highlight mutable identifiers via locals.scm