Add periodic version check

[joelspadin]

To let users know when a new version of ZMK CLI is available, it would be helpful to add a check on startup that gets the latest version from PyPI and prints a message telling the user to update. If possible, we should detect whether ZMK CLI was installed with pip or pipx and print an appropriate upgrade command, e.g.

# pipx
pipx upgrade zmk

# pip
python3 -m pip install -U zmk

This check should only be run periodically, e.g. the first time the program is run each day, so we don't waste time repeatedly checking the version. There should also be a setting to disable this, and possibly one to adjust the time between checks.