Add PlayerPrepareTeleportEvent

[ThatTransSky]

Continuing off of #12879, Adding PlayerPrepareTeleportEvent would allow plugin developers to better handle the case of disallowed cross-world teleports when the player has passengers.

There seems to be a need for some kind of a solution, even if a temporary one, since there's no way to handle this case as PlayerTeleportEvent doesn't fire at all if the player has passengers.

Relevant issues: #12093, #10168 & #10173.

Let me know if this PR is missing anything and I'll make any necessary changes!

[Cubicake]

I'm not a Paper contributor so my opinion is meaningless but this generally looks cool to me.

[ThatTransSky]

Ignore the force pushes, I misunderstood the original comment 😅

[Cubicake]

Set.of() is already immutable, you don't need to use ImmutableSet

[ThatTransSky]

Set.of() would make a Set<Set<TeleportFlag>> (since the teleportFlags parameter is already a Set<TeleportFlag) which why I got confused in the first place.
I can still switch to Set.copyOf() (I just forgot it was a thing).