We don't seem to expose any API that uses `char16_t` yet, but I added it anyway since we make the type info for it.
I didn't add anything for `wchar_t` because we are not making a type info so maybe we don't have a need for it yet, it could be added in the future.
To prevent breaking compatibility with the C# bindings, we ignore the `char32_t` metadata and still use `System.Int64`.