Directly Implement mlua::UserData on rbx_types::*

[krakow10]

I'm making a roblox emulator and noticed it would be convenient to directly implement mlua::UserData on types such as CFrame. My expectation is that this would be seen as a maintenance burden rather than a worthwhile feature, so I'm opening an issue.

[kennethloeffler]

I'm not against this, since it seems like it's common for consumers to want to expose rbx_types to Lua, mlua is widely used, and it wouldn't require a lot of ongoing maintenance. I'd accept a PR to do so, as long as it's gated behind a feature.

Before anyone starts work on this, they should review Lune's implementation and ensure that rbx-dom mlua feature will work for Lune with minimal changes, since Lune is the most prominent consumer of rbx-types that needs to expose rbx-types to Lua.