refactor(gdshader): use uniform scope node instead of literal matching

This avoids the need to refactor if other scope types are added. Maybe
helps performance as well.

runtime/queries/gdshader/highlights.scm

@@ -1,6 +1,4 @@
 [
-  "global"
-  "instance"
   "group_uniforms"
   "uniform"
   "const"
@@ -10,6 +8,8 @@
   "shader_type"
 ] @keyword
 
+(scope) @keyword
+
 [
   "if"
   "else"