WIP: add `jj config unset`

[pylbrecht]

Allow unsetting config values similar to git config unset.

$ jj config set --user some-key some-val
$ jj config get some-key
some-val
$ jj config unset --user some-key
$ jj config get some-key
Config error: configuration property "some-key" not found
For help, see https://martinvonz.github.io/jj/latest/config/.

Unsetting nonexistent keys will result in an error:

$ jj config unset --user nonexistent
Error: configuration property "nonexistent" not found
For help, see https://martinvonz.github.io/jj/latest/config/.

Closes #4458

Checklist

If applicable:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added tests to cover my changes