feat(local-notifications): Interruption level iOS

[markxoe]

Closes #659

I am guessing that some additions to the docs are missing and my implementation may not be the best, I do not work with Swift that often 😄

[davidemerritt]

+1 for this work - thank you - this is very important for our use case. We appreciate any movement on this.

[davidemerritt]

Any update on when this may be finished/implemented?

[davidofwatkins]

Thanks for starting this - this is great! Any word from any Capacitor maintainers about how we could move this forward?