Add --due-date and --clear-due-date to the edit command

[rameshbaskaran]

reminders add supports -d/--due-date, but reminders edit had no way to change a reminder's due date - only its title and notes. Changing a due date meant deleting and re-adding the reminder.

This adds to edit:

• -d/--due-date <date> - set a new due date, parsed by the same DateComponents natural-language parser add uses
• --clear-due-date - remove the due date entirely

Behaviour details:

• Setting a new due date clears existing alarms and adds an absolute alarm when the parsed date includes a time, mirroring add.
• Clearing the due date also clears alarms, so a stale notification doesn't fire.
• The two flags are mutually exclusive.
• edit now accepts a due-date change on its own; previously validation required new text or notes.

Tested manually against a scratch list on macOS: setting a date, setting a date with a time, clearing, combining a date change with new text, and both validation errors.

$ reminders edit Soon 0 --due-date "tomorrow 9am"
Updated reminder 'Some edited text'
$ reminders edit Soon 0 --clear-due-date
Updated reminder 'Some edited text'

Branched off main and independent of #107 (--repeat); no overlapping changes.