Add a JSON schema for Content Patcher include files

[eth0net]

There's currently no schema for Include files, even though the format is a subset of the main content file — Changes alone, without Format. content-patcher.json can't stand in for the same reason: it requires Format.

This adds content-patcher-include.json. It $refs content-patcher.json#/properties/Changes rather than duplicating it, so it shouldn't need maintaining alongside the main schema — I'd been using a hand-written copy of that subschema locally, and it had fallen out of date within a few months.

Thanks for publishing these — they've saved me a lot of time working on content packs. Glad to rename it, adjust the wording, or close this if you'd rather not carry another file.