Create SectionExitHandler interface

[takejohn]

Description

• Add an interface SectionExitHandler, having one method exit(Event).
• Change LoopSection to implement this.
• Change the 'exit' and 'return' effects to check if the enclosing nodes are SectionExitHandlers instead of checking for LoopSections.

This PR enables addons to add sections which don't need to hold loop counters doing something when exited.

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Target Minecraft Versions: any Requirements: none Related Issues: none

[APickledWalrus]

I'm not sure about the name. What do you think about something like ExitableSection

[APickledWalrus]

Thanks for this!