Feature request

Feature description
An event called when teleport warmups start.

How the feature is useful
This would allow other plugins to listen to the event and add their own functionality (like #2468).

Implemented in #2590.

#2590 doesn't implement this despite its name - it adds an event that is called between the end of the warmup and the teleport safety checks, which is... not particularly useful. I'm considering reverting this and adding two events in their place, a TeleportWarmupEvent (before the warmup) and a PreTeleportEvent (in place of that PR's event). The PlayerTeleportEvent implemented in #2590 is largely useless in its current form.