It can be useful to trace more positions, maybe rangeStart, rangeEnd can be useful, if we do #16292

1. Add a new doc (description: string) => {type: 'mark', description: string}
2. Make cursor alias of {type: 'mark', description: 'cursor'}
3. Deprecate cursor in v4
4. Remove cursor in v5

Alternatively, we can consider reuse label or use Symbol.