Recurring events

It was suggested that supporting recurring events would be a welcome addition to markwhen, so I added it.

The syntax is essentially the date range followed by every (duration) for (duration | # times):

March 2023 every other month for 12 months: ...
July 1 1999 every 3 weeks for 2 years: ...
May 15 8am every 5 minutes x30: ...
2022/2023 every 5 years for 10 times: ...

Other updates #

• Groups and sections now collapse events into a single line, so you can still see them
• The documentation have been updated
• VS Code extenstion updated to support recurring events
• Image export has been reintroduced, but only works on Chrome browsers and doesn't yet support recurring events.