Add `datetimeIn` for timezone offset

[meduzen]

datetimeIn(date, timezone, optionalPrecision) could take a local time and shift it to another timezone.

Use case: “what time is it now in [somewhere far away]?”

How we could make it work:

1. Read the UTC values of the provided Date object.
2. Do something with these values (sorry, should go to bed) and use Intl.DateTimeFormat to transform the timezone to the specified location, e.g.:

new Intl.DateTimeFormat('en-GB', {
  dateStyle: 'short',
  timeStyle: 'long',
  timeZone: 'America/New_York'
}).format(date); // '03/05/2021, 17:28:22 GMT-4'

3. Compute timezones difference by comparing the GMT values and/or reparsing the output of 2.
4. ??

(Also, check what Moments and others are doing.)

[meduzen]

Issue updated.