Add `jj debug snapshot` command and use it in trigger

[fowles]

The command only takes a snapshot and avoids other overhead, so it can be used as a target for the watchman trigger that gets installed.

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)
• [x] I have added tests to cover my changes

[fowles]

I opted not to put anything in CHANGELOG.md since this is an internal command. Happy to add something if others disagree.