Handle Union as input arg

[francisbouvier]

We handle union as return type (ie. Zig -> JS) but we also need to handle union as input type (ie. JS -> Zig).

Use case: https://developer.mozilla.org/en-US/docs/Web/API/CharacterData/after

Here node input argument can be either a proper Node or a simple string. Zig unions should be the right type to handle this case.

Links to #54