Add support for tagged userdata and userdata destructors

[nurpax]

Luau doesn't support the usual metatable __gc method, instead userdatadtors should be used. There's more information available about these differences here:

luau-lang/luau#251 (comment)

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Pushing patch for FYI and review. This is probably the last of my pending Ziglua changes that I think make the Luau support reasonably complete to be used in let's say a game engine.

[nurpax]

This patch is also ready for review @natecraddock.

Couple of notes:

• CUserdataDtorFn -- this passes an anyopaque pointer to the dtor callback. Maybe there's a cleaner way to do that?
• I mirrored a couple of api_check asserts from Luau into the Zig bindings. Reason? Hitting asserts on Luau side leads to really confusing errors at least on Windows. I expect giving an out of range tag index to be a probable mistake, so this is just to make diagnosing the mistake easier. OTOH, I don't want to make this return an error, since the original Luau API doesn't do that either, and IMO it's a little silly for the bindings to start doing extra error checking.

The functionality added by userdatadtor is IMO required functionality in the Luau version, as Luau doesn't support the usual Lua __gc metatable function.

[nurpax]

I'll rebase since the other change went in

[natecraddock]

Thanks again for contributing! Just a couple of style things I pointed out