Create JSON diffs for use in GitHub Actions

[tconbeer]

Related to tconbeer/sqlfmt-docs#11

SQLFluff outputs JSON annotations for use with another GHA to annotate PRs. This is really nice! We should do this too. More.

Option could be either --diff-json or --diff --json. JSON should print to stdout for redirection purposes, e.g.,

sqlfmt $(git diff main --name-only) --diff --json > annotations.json`

Need to research how to format the annotations to show the desired change (a la code review suggestions)