GDScript: Consolidate behavior for assigning enum types

This makes sure that assigning values to enum-typed variables are
consistent. Same enum is always valid, different enum is always
invalid (without casting) and assigning `int` creates a warning
if there is no casting.

There are new test cases to ensure this behavior doesn't break in
the future.

modules/gdscript/gdscript_warning.h

@@ -70,6 +70,7 @@ public:
 		REDUNDANT_AWAIT, // await is used but expression is synchronous (not a signal nor a coroutine).
 		EMPTY_FILE, // A script file is empty.
 		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.
 		WARNING_MAX,
 	};