Add $schema to JSON for better VSCode support

[A5rocks]

I'm not completely sure adding $schema is fine, as this changes two (or more, because I may be missing spots) external things:

• adds $schema when you get a list's info
• maps the $schema ID to the invalid list ID ../schema/legacy.json for stats posting due to how legacy mapping works -- but this is probably fine since the user could have just provided that URL anyways and $schema is an invalid list ID.

[MattIPv4]

👀 Does this require a small API change first to drop the $schema prop from the responses in /api/lists, /api/lists/:list and /api/legacy-ids?

[A5rocks]

It could do that (and I would be fine adding that) but really clients shouldn't care about the extra key.

[MattIPv4]

Clients probably wouldn't care about the extra key, but it would mean that the API schemas would be incorrect as the extra key would be present in the responses. I also don't think it makes much sense to be returning this key in the API responses as it's a relative path that wouldn't point to anywhere useful relative to the API.

[A5rocks]

botblock/api-worker#4

[MattIPv4]

LGTM