Create `shard.yml.schema`

[nobodywasishere]

It would be useful to provide a JSON schema for the shard.yml, allowing for IDE validation and documentation while editing the shard.yml. Ideally, this would then be added to JSON Schema Store for direct IDE support. I can implement this and create a PR if it would be accepted.

[straight-shoota]

That would be really lovely. 😍

shard.lock and lib/.shards.info are also standardized YAML formats. However, they're only meant to be written automatically and not expected to be edited. So I suppose a schema wouldn't be much useful there.