Entry schema, validator and CI check for listed mods #9

A listed mod is mods/<id>.json. Needs a schema (what the author files; what a listed entry records: EchoTools fork, pinned release tag, hashes, signer status), a validator and a CI check. Done when the check runs on every PR with tests that have negative controls. Resolved by the PR that closes this issue. Part of the plugin-signing design (author submits once, each listed plugin points at its EchoTools fork and release).