feat: refactor with templating support

[avivkeller]

Fixes #14
Fixes #5 - Custom information, like a Slack channel, can be provided via the template.

I'll admit... this is a pretty substantial PR. But it gets the action ready for use by the nodejs/web-team, especially since there’s still no ETA on Node.js’s own generator.

This PR includes:

• Templating support, allowing us to specify invitees for nodejs/web-team.

• Removal of DST offset handling:

  • During testing, all times ended up being an hour off.
  • More importantly, I don’t think DST adjustments are expected behavior. If a recurring meeting is set for 12:00 PM, it should stay at 12:00 PM, regardless of daylight saving time.

[bmuenzenmeyer]

DST offset, this really emphasizes how important it is to be able to test this - cause in my experience (when this was more of a CLI than an Action, I had off-by-one errors the other way). it felt related to DST and rrule parsing.

I'd like to look more closely into this, but have low capacity and this also emphasizes how hard it is to test and iterate on