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!

[sovdeeth]

It's not safe to cast like this. You're accepting any entity (using entity in the pattern) without checking whether it will truly be a Leashable.

[georgereuben]

I see. SInce I cannot add Leashable in the pattern, the other solution I have tried now is to update the convert method to accept Entity objects and then add a check within the convert method. Please let me know if this works.

[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