WorldGuard: markerSetName flag

It would be nice to have a custom flag to overload the `markerSetName` value, so different types of regions can have their own set of markers in the markers list:
<img width="317" height="364" alt="Image" src="https://github.com/user-attachments/assets/948be2f6-f5d5-490b-ab89-98b7eb9495ee" />