Edit config files with a temp editor and add some validation

[leeavital]

Fixes https://github.com/martinvonz/jj/issues/3905

This solves a problem where jj config edit drops you straight into an editor and allows you to save an invalid toml file. If you mess up your toml syntax, jj cannot launch (even jj config edit!)

This implements a pattern I've seen in tools like crontab -e or visudo where you get dropped into a temporary file editor and the tool does a bit of sanity checking before copying over the contents of the temporary file into the actual file.

Code wise: this reuses a utility function which takes care of the heavy lifting for creating a temporary file and launching an editor. I put all the validation in a new function in cli/src/config.rs, which seemed a bit like overkill but I figured the details of the config format being toml shouldn't leak into cli_util or command/config

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

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[yuja]

According to the issue comment, I think temporary backup + restore is preferred over editing temporary file. https://github.com/martinvonz/jj/issues/3905#issuecomment-2175297503

visudo has to be really strict (so the system will never get broken), but we wouldn't need a guarantee of that level.