/mount command

The command should work like the /spectate command. But it will be possible to mount every entity. This might would be really useful.

There should be additional tags for better use in data packs:

- HasMount:1b (is set to 1 when an entity is riding the target entity)
- IsMounted:1b (is set to 1 when the target entity is riding another entity)
- Riding:[{*entity*}] (this tag should return so that you can detect who is riding who)
- Passengers:[{*entity*}] (this tag should stay so that you can detect who is riding who)
- Controllable:1b (is set to 1 if the riding entity is supposed to be able to control the mounted entity)