Remove calendar-cli and kal from plann

[tobixen]

As of 2023-01-08, the old project calendar-cli was forked into the new kal project which was shortly afterwards renamed to plann to avoid name conflicts. Lots of work is needed to make the split clean.

• Remove all legacy code from the plann project (search for "calendar-cli")
• Search for all instances of "kal" (and rename to plann if appropriate)
• Make sure the documentation is valid for plann, not calendar-cli
• The directory should be renamed from calendar-cli to plann. setup.py needs to be fixed, etc
• It should be pushed to pypi - but there is an issue there as someone has already published something under that namespace on pypi, I need to figure out how to resolve that.
• Old github issues from calendar-cli should be looked through, anything there that is plann-relevant?