diff: add a file-by-file variant for external diff tools

[fowles]

Checklist

If applicable:

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

[yuja]

Just for reference, there are some discussion in https://github.com/martinvonz/jj/issues/3696 (I haven't reviewed the implementation yet)