✅ Make testing easier

[hgwood]

Some ideas to improve the experience of developing this tool:

• Add a dry run mode: runs normally except that it logs what should be written to Google Calendar instead of actually writing it.
• Add a flag-as-test mode: runs normally but flags events written to Google Calendar as tests (by adding [TEST] in the summary for example)
• Allow for the target calendar to be overridden using a calendar ID in an environment variable so that all events are written to it instead of the primary calendar of any given user. This target calendar must be writeable for all users.
• Extract a pure function from the current code base that can map a collection of Alibeez leaves to a collection of { action, eventId, eventBody } triplet so that testing this mapping becomes trivial and modifying it is safer.