Use case-insensitive key comparison for cache keys

[BytewaveMLP]

Following the discussion in #659, this PR introduces a case-insensitive cache key comparison function to compare the cache key returned from @actions/cache versus the calculated cache key for the purposes of partial-hits.

This implementation follows that which is used in actions/cache. Unfortunately, that implementation isn't in the accompanying Node package, so I've copied it here.

Closes #659

[eregon]

Sorry for not reviewing this sooner, I think was unsure whether this was safe enough.
Since this works on GitHub Actions and act is not tested (in this repo) and so not supported I think this is something to fix in act instead.
OTOH it seems a safe enough change, and it seems nice if people can use act locally to run GitHub Actions.
It seems many places are already doing the same: https://github.com/search?q=cacheKey.localeCompare&type=code

I'll merge it, but if it causes any issues I'll revert.