Add a json schema for the configuration

[jneem]

Automatically derives a json schema from the configuration struct. This uses the schemars crate which checks the serde annotations to make sure that, e.g., names are correctly renamed to kebab-case.

I added a little binary to generate the schema as a text file. I'm not sure what to do with the result (e.g. maybe it should be automatically built and uploaded somewhere as part of a release).

This should basically fix #2894, as taplo apparently supports autocompleting from json schemas.

[pascalkuthe]

there was a past PR for this where it turned out that achieving generally correct schemas requires a bunch of effort.

Since the TOML config will be removed in the future I don't think it makes sense to invest that effort: #3901 (comment)

[pascalkuthe]

#6174 for reference