Split rule-syntax-check into generate-rule-schema and validate-rules

[noisysocks]

Expands on #613 and adds 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 writing and editing rules:

https://json-schema.org/

[muodov]

Looks nice overall, just a few maintainability concerns

[muodov]

ts-node is a new dependency that we'd need to consider carefully. For now, sticking with plain JS for CLI scripts and using JSDoc comments for typing would be more consistent with other DDG repos.

[noisysocks]

Is there a particular concern you have with ts-node? get-text-for-xpath (#386) is an existing script that uses it. I think TypeScript is nice for scripts as it gives you a little bit of extra confidence in an environment that doesn’t usually have unit tests. But it's not a strong opinion, happy to defer to you on this.

[muodov]

is it possible to point to a local file?

[noisysocks]

Seems to work fine in my testing. It might be IDE dependent. We can do local refs for now until it’s a problem or we have a need for third party rules. 6cc349d

[noisysocks]

Actually, hm, I’m having second thoughts. Using ”../schema.json” as the $schema means that the combined rules in rules/rules.json have invalid $schemas as they’re in a different directory. It seems likely to me that these rules will move around as the project evolves and we explore remote configuration etc. Using an absolute URL ensures that rules are atomic: you can move them, copy/paste them, whatever, and they remain valid.

I reverted 6cc349d.