Helper methods to invoke code if object is a `something` or else invoke a fallback method

[Earewien]

Add GTraitscore methods:

• if_is_a(a_trait:Script, object:Object, if_callable:Callable, call_deferred:bool = false) -> Variant
• if_is_a_or_else(a_trait:Script, object:Object, if_callable:Callable, else_callable:Callable, call_deferred:bool = false) -> Variant

Automatically generate trait specific helpers methods in GTraits