rust: fix highlight corner-cases

* add punctuation highlights for commas as in function parameters
* remove stray `variable.parameter` highlight
    * I couldn't find any regressions from this and it fixes an
      edge case I ran into (but sadly did not record 😓)
* highlight `fn` as `keyword.function`
    * the theme docs have `fn` as an example so it seems fitting

runtime/queries/rust/highlights.scm

@@ -57,6 +57,7 @@
   "::"
   "."
   ";"
+  ","
 ] @punctuation.delimiter
 
 [
@@ -95,8 +96,6 @@
   value: (identifier)? @variable
   field: (field_identifier) @variable.other.member))
 
-(arguments
-  (identifier) @variable.parameter)
 (parameter
 	pattern: (identifier) @variable.parameter)
 (closure_parameters
@@ -141,7 +140,6 @@
   "mod"
   "extern"
 
-  "fn"
   "struct"
   "enum"
   "impl"
@@ -160,6 +158,8 @@
   "async"
 ] @keyword
 
+"fn" @keyword.function
+
 (mutable_specifier) @keyword.storage.modifier.mut
 
 (reference_type "&" @keyword.storage.modifier.ref)