Revisiting nil semantics in Variant and TypedArray

[elijah-semyonov]

This PR contains the following changes, all of them are related to nil-correctness.

1. Godot Variant with gtype == .nil should never surface in the user API. It will be always represented as Swift nil of Variant?. This allows direct integration of Swift nullability checks into Variant processing. We shouldn't have places where we have Variant?.some with Godot nil inside. This is accompanied with a lot of small code changes everywhere in the code to support this logic.
2. All places that previously used Variant will now be Variant?.
3. Improved ObjectCollection: TypedArray of objects can now work with nils (just like it does in GDScript).
4. Godot Nil builtin class is omitted from translation.
5. Some instrumentation for macro testing to make their updates less painful (generating a new macro test body and pasting it into clipboard, Mac only for now).
6. New API inside Arguments to streamline gracious failing when Godot calls are done into @Callable or @Exported. It shouldn't crash the game/editor anymore and print errors instead.
7. Update of MacroCallable and MacroExported for newer infrastructure and gracious failing if Godot calls into Swift-exported code with wrong arguments instead of crashing.
8. Lots and lots of tests to cover all the changes.