This fixes#4336 crash at login about mcl_mobspawners. Also adds an API call, `mcl_mobs.register_conversion` for converting one mob into another and updates rovers and stalkers to use this API call.
Reviewed-on: VoxeLibre/VoxeLibre#4337
Reviewed-by: the-real-herowl <the-real-herowl@noreply.git.minetest.land>
Co-authored-by: teknomunk <teknomunk@protonmail.com>
Co-committed-by: teknomunk <teknomunk@protonmail.com>