Define config TOML/JSON schema

[dbarnett]

Can be used with tools like taplo-lsp to show hints & validation in editors/IDEs. Won't apply automatically to config files until it's submitted to schemastore.org, but in the meantime it can be used via a schema directive
(https://taplo.tamasfe.dev/configuration/directives.html#the-schema-directive) or other manual config mechanism.

Context in #879.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have added tests to cover my changes

[dbarnett]

See https://json-schema.org/ for details on how JSON schemas are interpreted.

FWIW, how I'm using this schema locally for now is to symlink REPO/src/schemas/json/jj.json to ~/.config/jj/_schema.json and then add this directive to the top of my ~/.config/jj/config.toml: #:schema ./_schema.json.

[dbarnett]

Resolved all comments.

I don't know that a CHANGELOG update makes sense for this one since it's not explicitly hooked up yet as a jj feature, just a file that's in the repo if you know how to make use of it. Same for README/docs. And for tests, it's awkward to test the schema itself, but might make sense to hook up to some kind of testing eventually.

[martinvonz]

I don't know that a CHANGELOG update makes sense for this one since it's not explicitly hooked up yet as a jj feature, just a file that's in the repo if you know how to make use of it. Same for README/docs. And for tests, it's awkward to test the schema itself, but might make sense to hook up to some kind of testing eventually.

Agreed. That checklist is meant as a reminder (particularly for me, because I've often forgotten to update those things :))

[martinvonz]

Looks good. Feel free to merge.