`$schema` in messages

We could start to add `$schema` URLs to generated JSON

For now this would just suffice to make them more comprehensible/validatable, but going forwards it could allow a schema versioning mechanism

The issue mentions generated JSON and the Cucumber messages protocol, but no files, tests, or entry points. Start by locating how message JSON is generated and how schemas or versions are currently represented; done would require an agreed `$schema` URL approach that makes the generated messages validatable and supports the proposed future versioning mechanism.

Written by the indexing model from the issue text.