Validate JSON rules

[muodov]

I borrowed some code from the remote config repo to validate JSON files against the existing TS types

[noisysocks]

Very nice! Love it.

I think we could go a even further and split this script into two scripts:

1. one that generates a JSON schema file, which we commit to the repo, and;
2. one that validates our rules against the JSON schema.

This way we can reference the committed JSON schema using $schema at the top of each rule and developers get autocompletion and hints in their IDE when editing rules.

I’ve done exactly this over in #615 which has your PR set as the base. What do you think?