Support lua_getthreaddata and lua_setthreaddata

[rhit-sashiks]

Luau exposes lua_get/setthreaddata for setting thread level data. Would be nice for mlua to support as you could then store data like a delay queue key etc in a thread directly without needing to use its pointer as a hash etc. (e.g, Thread::get_data(&self) and Thread::set_data(&self)