Create jsonschema for configuration file

[zidhuss]

This PR adds a JSON Schema that describes the .swift-format configuration file.

When this schema is referenced, It serves as a form of documentation and provides auto-completion and validation for configuration options (in editors that support it).

Ideally you'd want to generate this from the code as done in #615. However the documentation for the other configuration options isn't automated from the comments leading to out of date documentation (#455).

My aim is to get this schema in the schema store so that editors automatically associate .swift-format files with it.

[ahoppen]

Sorry for the late review @zidhuss. Adding a reference for the .swift-format file is great!

I assume you wrote the schema by hand, right? I think it would be great if we had a script to make sure it’s in sync with Configuration.md. Maybe we could even generate Configuraiton.md based on the JSON schema. Could you be interested in writing such as script (ideally in Swift but really any scripting language would be fine)?

[allevato]

It would be great to generate this from the doc comments in the Configuration struct (and the other structs that it contains) like you mentioned was done for rules in #615, and also generate the Markdown from that (or alternatively, just abandon Configuration.md in favor of DocC comments). Otherwise, if we just check in a hand-written version of this file, it will go stale the same way the Markdown has.