improvement: allow modifications of supertree in Zipper.within/2

[zachdaniel]

We have our own version of within/2 in igniter for unrelated reasons, but I thought this improvement might be welcome.

We have a situation where we want to both:

1. preserve the original location
2. modify the "surrounding code".

By using the returned zipper's supertree (if present), we allow this kind of modification.

[doorgan]

This sounds good! Can we add a test for the use case?

[zachdaniel]

Can do :) I will add one next week.

[zachdaniel]

@doorgan done :)

[doorgan]

Awesome, thank you! 💜