VM: Add functions to get/set inline destructors of userdata

[Nerixyz]

This adds

lua_InlineDestructor lua_toinlineuserdatadtor(lua_State* L, int idx);
void lua_setinlineuserdatadtor(lua_State* L, int idx, lua_InlineDestructor dtor);

They get/set the destructor for userdata with an inline destructor (UTAG_IDTOR). Setting the destructor after the userdata has been created is more important here. The use-case is fallible construction of the userdata:

void *mem = lua_newuserdatadtor(L, sizeof(T), [](void*mem){
    std::destroy_at<T>(static_cast<T*>(mem));
});
T *udata = static_cast<T*>(mem);
// Try to construct T. If this fails (e.g. arguments can't be converted), we shouldn't
// invoke the destructor, so we should unset it.

I found this when trying to get the sol2 Lua bindings to work with Luau. There, the userdata is constructed first and the arguments are matched later. In case of overloaded functions, there might not exist an overload, so we'd error out and drop the userdata. We can only set the destructor once we constructed the value inside the userdata.