Add list versions JSON

- Avoid Ruby startup for inventory-only JSON output.
- Emit formulae and casks from fast filesystem state via `jq`.
- Include nullable linked, opt-linked and pinned versions.
- Reject invalid JSON option mixes in Bash before Ruby starts.
- Source `list.sh` in tests to avoid checkout `Cellar` state.
- Document `--json` for manpages while Ruby fails if reached.
- Cover combined, filtered, empty and missing-`jq` cases.