Update the schema.json to use references?

@rictic tagging you as you implemented the initial version in #125 As I was FAFOing trying to figure out potential strategies for #1188, I found the current schema a little challenging to work with. I reworked it so it uses references and it accurately requires a `command`, `dependencies` or `files` property. So far, after adjusting a few error messages to correct for the exact error messages, all the tests are passing with flying colours. If I were to implement all the other requirements and variations, would there be interest in a PR to update the schema.json? Progress so far: https://github.com/TyIsI/wireit/blob/feat/schema-file-update/schema.json Let me know!