Add support for Unicode identifiers in GDScript

This is using an adapted version of UAX#31 to not rely on the ICU
database (which isn't available in builds without TextServerAdvanced).
It allows most characters used in diverse scripts but not everything.

modules/gdscript/gdscript_warning.h

@@ -78,6 +78,7 @@ public:
 		SHADOWED_GLOBAL_IDENTIFIER, // A global class or function has the same name as variable.
 		INT_ASSIGNED_TO_ENUM, // An integer value was assigned to an enum-typed variable without casting.
 		STATIC_CALLED_ON_INSTANCE, // A static method was called on an instance of a class instead of on the class itself.
+		CONFUSABLE_IDENTIFIER, // The identifier contains misleading characters that can be confused. E.g. "usеr" (has Cyrillic "е" instead of Latin "e").
 		WARNING_MAX,
 	};