stan: Create `stan.json` alongside `stan.html`

[srid]

Another thing worth considering: add a flake check that will fail if project health is not 100%. Poeple can always add excludes to their stan config if there's a good reason to reject certain suggestions.

Originally posted by @srid in #384 (comment)

Demonstrate this on https://github.com/srid/haskell-template

[srid]

Not sure if this should be in haskell-flake per se. But a demonstration outside of it would be a good start.

[srid]

But a demonstration outside of it would be a good start.

That said, having haskell-flake produce a stan.json (through stan --json-output) would be necessary for this.