Refactor typeregister's BUILTIN_ENUMS => BUILTIN.enums

This will allow us to add more buildin types to this thread local
in follow-up patches, to reduce the number of thread local slots.

internal/compiler/expression_tree.rs

@@ -259,7 +259,7 @@ impl BuiltinFunction {
             }),
             BuiltinFunction::ColorScheme => interned_type!(Function {
                 return_type: Type::Enumeration(
-                    crate::typeregister::BUILTIN_ENUMS.with(|e| e.ColorScheme.clone()),
+                    crate::typeregister::BUILTIN.with(|e| e.enums.ColorScheme.clone()),
                 ),
                 args: vec![],
             }),