Extend render_json! macro

- [ ] Support passing in a JSON-structure with substitutions in `render_json!`
- can most likely pass all tokens down to serde_json's `json!` macro

https://github.com/killercup/output-rs/blob/d28762e34d20976812542ab8dbd76fc3f1318919/src/json.rs#L63

Start with the linked src/json.rs implementation of render_json! and compare its macro inputs with serde_json's json! macro. Confirm how JSON structures with substitutions should be accepted, then verify that render_json! supports that syntax and preserves the expected output.

Written by the indexing model from the issue text.