"boardable" property

[Adil]

Entities currently have [rw] `operable` property. It prevents opening the interface of the entity.
However, it might be useful sometimes to prevent the ability of player entering a certain vehicle.
`operable` doesn't cover that, and this might actually be handy in some scenario of sorts.

[Supercheese]

While this would slightly simplify matters, you can nonetheless achieve the same result by monitoring the on_player_driving_changed_state event and ejecting any player who enters a certain vehicle.

For example: https://github.com/GopherAtl/nixie-tube ... #L362-L368