feat(highlights)!: enforce documented captures (#6232)

Problem: Allowing undocumented "secret" (sub)captures makes it harder
to write comprehensive colorschemes and catch inconsistent captures.

Solution: Only allow captures listed in CONTRIBUTING.md. Add useful
(cross-language) subcaptures and drop language-specific or too niche
ones.

Follow-up: Adding further `*.builtin` captures and changing queries to
use them.

Language-specific subcaptures should instead be added in user config or
a custom language plugin.

queries/squirrel/highlights.scm

@@ -64,15 +64,16 @@
 ] @type.qualifier
 
 ; Variables
-(identifier) @variable
+[
+  (identifier)
+  (global_variable)
+] @variable
 
 (local_declaration
-  (identifier) @variable.local
+  (identifier) @variable
   .
   "=")
 
-(global_variable) @variable.global
-
 ((identifier) @variable.builtin
   (#any-of? @variable.builtin "base" "this" "vargv"))
 

queries/squirrel/locals.scm

@@ -67,9 +67,9 @@
 (var_statement
   "var"
   .
-  (identifier) @local.definition.variable)
+  (identifier) @local.definition.var)
 
 (local_declaration
-  (identifier) @local.definition.variable
+  (identifier) @local.definition.var
   .
   "=")