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