WIP: Create an example API.

[matts1]

This PR is meant to trigger a discussion on what the specific design of the API should look like.

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

[matts1]

I believe that the best command to start with would probably be jj log, since the APIs for that are probably ~80% of what's needed for APIs (I can run `jj describe -m "..." myself pretty easily, for example, but determining what the commit message currently is so I can modify it would be hard).