provide addInTimeZone function

[dr-skot]

Love this library. One feature it's missing that I've needed from time to time is a timezone-aware version of the date-fns add function.

Time zones matter in date arithmetic because of daylight savings. If your base time is 2022-11-05T06:00Z, for example, and you want to add one day,

in LA it's
11pm Nov 4 + 1 day === 11pm Nov 5 === +24 hrs

but in Halifax it's
3am Nov 5 + 1 day === 3am Nov 6 === +25 hrs

because the clocks fall back 1 hr (in both places) at 2am Nov 6.

So here's a proposed addInTimeZone function to handle that. Plus some tests.

I tried yarn build to generate the typings files but no joy. I guess some more configuration twiddling is needed to make that work.

Hope you find it useful!

[marnusw]

Thank you @dr-skot, I'll take a look in the near future.