* update.py: introduce subparsers for plugin updaters
This is preliminary work to help create more powerful plugin updaters.
Namely I would like to be able to "just add" plugins without refreshing
the older ones (helpful when github temporarily removes a user from
github due to automated bot detection).
Also concerning the lua updater, we pin some of the dependencies, and I
would like to be able to unpin the package without editing the csv
(coming in later PRs).
* doc/updaters: update command to update editor plugins
including vim, kakoune and lua packages
Co-authored-by: figsoda
351cec5db3