I just saw [this PR](https://github.com/exercism/gleam/pull/114) utilizing `files.invalidator` and it doesn't seem to be documented in https://github.com/exercism/docs/blob/43a2d226da5e2c8631de8d5006adb9c60ccf877b/building/tracks/config-json.md. Is this on purpose or should this be added?