✨ Allows returning a function from init to run after unmount

[ekwoka]

This PR enables the common behavior in component based UI systems, where an onmount function can return a function that will then be called at cleanup.

This would be an alternative to destroy, especially for cleaning up any effect made in init but with the benefit of not needing to manage storing data outside of the closure.

This also handles async init methods that would return a promise that resolves to the cleanup

[calebporzio]

This makes sense and I appreciate the contribution, however as it is a fairly impactful change I'm going to hold off. Thanks again

[ekwoka]

I know you mentioned possibly adding it in the past, but wasn't sure if it was backburnered or you decided against it.