Luxon: Expose Zone objects in the API

[farnoy]

First off, thank you for this amazing library. I use it a ton and it's a godsend.

I'm running into perf limits when doing a lot of setZone() operations. These seem to be particularly slow on Chrome and I already reported it upstream https://bugs.chromium.org/p/chromium/issues/detail?id=928098

When I profile performance, the call stack setZone - > normalizeZone -> IANAZone -> isValidZone occurs a lot and takes up a lot of time. Can you expose the Zone API to the user so that we can look up zones once and use it with many timestamps? I think it would speed up these operations a lot, because the constructor of Intl.DateTimeFormat can be expensive (and on top of that, about 8x slower on Chrome currently than other browsers).

I noticed in the manual that we can already define Zone objects, but it's supposed to be used for custom stuff only? I think this could be extended to expose IANA zones with this interface.

[farnoy]

I dug deeper and the IANAZone object is exported and has a constructor that seems to work fine. So no changes are required I guess, except for the manual/documentation? Is this supposed to be unstable, private API or can we use this and expect stability?

[icambron]

It's intentionally public but it is undocumented. It was originally private and I got basically this same request and we started exporting it, but we didn't update the docs. Would love those docs though!

[icambron]

Documented in b12d7fa