Feature: Convert microformats2 h-event to iCalendar (ICS)

[gRegorLove]

In some recent IndieWeb dev chat (example), there has been an idea for Granary to support converting microformats2 h-event to iCalendar. This issue is to gauge interest and discuss further.

Prior work:

• https://h2vx.com/ converts classic mf1 (not microformats2) hCalendar to iCalendar
• PHP library: https://github.com/gRegorLove/mf2-to-iCalendar
• ...

[capjamesg]

I would love this feature!

[snarfed]

First pass at this (!), generated by Claude Code, unreviewed: main...claude/add-icalendar-support-TJKUz

cc @btrem @tantek @kevinmarks. I told it to review the H2VX code heavily and use its examples carefully. 🤷