Update ExprLeashHolder to use Leashable instead of LivingEntity

[georgereuben]

Problem

Currently none of the Leash related syntax work with boats meaning you will have to use Reflect and paper syntax instead.

Solution

Updated ExprLeashHolder and EffLeash to use Paper's Leashable instead of the LivingEntity interface.

Testing Completed

Tested with quickTest, skriptTest and Junit tasks

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Completes: #8239

[sovdeeth]

Would also benefit from some tests, if possible. Test that you can leash a living entity, a non-living entity, and ensure that a non-leashable just returns null and doesn't throw exceptions. See the README in the tests directory for guidance on adding a test file!

[georgereuben]

Would also benefit from some tests, if possible. Test that you can leash a living entity, a non-living entity, and ensure that a non-leashable just returns null and doesn't throw exceptions. See the README in the tests directory for guidance on adding a test file!

Okay will do

[sovdeeth]

looking great

[sovdeeth]

Suggested change

	if(entity instanceof Leashable leashable) {
	if (entity instanceof Leashable leashable) {

[sovdeeth]

should assert that the leash holder of cow is properly set after each line

[georgereuben]

yes this is just a basic template, I was having some trouble with unrelated tests failing after adding this test. Probably some issue with cleanup and state restoring. I'll be adding assertions soon

[sovdeeth]

Looks like Leashable isn't available on all versions we support, so you'll have to do some Skript.classExists() checks to make sure it's safe to use it.

java.lang.NoClassDefFoundError: io/papermc/paper/entity/Leashable
	at ch.njol.skript.effects.EffLeash.execute(EffLeash.java:64) ~[Skript.jar:?]