mbsync: Ask before making any changes

^{Originally posted by **Maxr1998** January 23, 2022}
When running `beet mbsync`, the command should print the proposed changes, and then ask for approval, similarly to `beet modify`.

That's because changes on MusicBrainz may not always be expected or wanted.

Always running the command with `-p` first, checking the output, and running it again without the flag is tedious (and may even introduce race conditions if something gets edited in between, even if unlikely).

For the old behavior, a `-y` flag could be introduced, or a configuration file option.