Fix loop memory leak

[TPGamesNL]

Description

Fix a memory leak in loops by removing from the Loop#current map after the loop is done.
The memory leak is caused by some plugin storing an instance of the event used, so the WeakHashMap entries aren't removed.

Currently, if the stop / return effect is used within the loop, the memory leak may still occur.

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Target Minecraft Versions: any
Requirements: none
Related Issues: #2337 #2877

[TPGamesNL]

Loops are bound to the Event instance, just like local variables, so the best way to fix this is to include it in the methods used to store/restore local variables (Variables.removeLocals and Variables.setLocalVariables) (although those methods should be renamed, and return some object that includes both the VariablesMap and the loop stuff)

[TPGamesNL]

Assuming there is a strong reference for the Event instance (even when the event has long passed), I see two ways to fix this:

1. Explicitly tell the Loop to remove the map entries for the event when:
   1. The loop is over.
   2. EffExit is used (one that exits the loop)
   3. EffReturn is used.
2. Tell all Loops to remove map entries, when TriggerItem.walk / Trigger.execute ends. If we choose this, any async effects / delays will corrupt loops and delete the current loop-value (similar to what used to happen to local variables), unless we specifically add a way to restore them (as with Variables.removeLocals and Variables.setLocalVariables).

If we assume Event instances are GC'd at some point, this has already been fixed with #3860.

[TPGamesNL]

After discussing with devs, we chose the first option