Improve performance when validating the same timezones many times

[billthornton]

Hello 👋

Firstly thanks for this project, it's much needed.

When upgrading to the latest version we found a reduction in performance where we were using utcToZonedTime() on many dates.

It looks like the improvement to use Intl.DateTimeFormat (in #129) for validating timezone strings came with a performance cost.

This PR looks to cache valid IANA timezones to improve performance when validating the same timezone many times.

For example, calling tzParseTimezone 100 times with the same America/New_York timezone, this PR takes it down from ~17-19ms back down to 3-4ms.

The other file changes look just from the linting being run on the file.

Thanks!

[marnusw]

Thank you @billthornton. I think using an object cache will be slightly more performant than the linear array search. (Although it's probably just academic.) I've added that optimisation and it will be released in an upcoming version.