Add support for dtend and floating dates

[simonratner]

PR for local review only; will issue PR to origin repo after.

[nickawilliams]

LGTM once the duration/DST issue is sorted.

[simonratner]

once the duration/DST issue is sorted

Actually i belive it is correct; added a test to prove it. Based on this, rrule treats all js dates as zero-offset dates (it ignores the timezone offset and expects all input to be zoneless-UTC). Output is also zoneless-UTC that should be assumed ot be in the local timezone, and must be converted using luxon or similar if you want the output in a particular timezone (or actual UTC).

The end effect of all this is that DST has no effect. The duration of the event is calculated on UTC dates which have no timezone and hence no DST, and they do not end up affecting the output either because the output is returned as local time.

[simonratner]

Btw, reading through jkbrzt#322 and the issue linked from there, there seems to be little chance of getting this merged upstream. DTEND is considered "out of scope" for RRULE.

[simonratner]

🛑 This PR should not be merged, see instead add-dtend-xxxx pre-built branches based off this.

[andjiev]

@simonratner what happened with this PR? As I can see from the rrule opened issue, it says feel free to send a pull request for this to get it included.

[simonratner]

This code is in active use. I'll send a PR.

[simonratner]

See jkbrzt#421.