[FEATURE] Enhance checkpoint functionality

[diademiemi]

Checkpoints should be aware of linked lines and have an optional teleport point which linked lines can use.

When a checkpoint is defined with ln line a_finish_line checkpoint add, it is given a number that is visible in ln line a_finish_line info and in tab completion.

This feature expands on that and is only available if there is a linked start line. (Syntax taken from draft for 2.0.0 #20)
ln line a_finish_line checkpoint 1 setteleport
If there is a checkpoint defined with a teleport point, when a start line teleports you because of an illegal area, it will teleport you to this checkpoint teleport point instead.
There should also be a command for administrators to teleport to this
ln line a_finish_line checkpoint 1 tp
The command to unset the teleport point should be
ln line a_finish_line checkpoint 1 removeteleport

[diademiemi]

There should also be a way to redefine checkpoints, and adding a checkpoint should have an option on where it should be on. Checkpoints should also have an optional argument to insert them into an existing list.
ln line a_finish_line checkpoint add before 3
ln line a_finish_line checkpoint add after 2