disalow splitting of closed roundabouts when they are part of a route

these cases are not covered by the splitting algorithm at the moment:
* typicall, only a part of the roundabout should be part of the resulting route
* when multiple routes meet at a roundabout coming from different angles, the roundabout must be split at any of those entry/exit point simutaneously, such that all routes remain correct

as a workaround, a mapper can remove the route relation(s) from the roundabout, split it accordingly and then re-add the respective parts to the relation(s) again.

data/core.yaml

@@ -500,6 +500,7 @@ en:
           other: "Split {n} features."
       not_eligible: Lines can't be split at their beginning or end.
       parent_incomplete: This line cannot be split because it is part of a larger relation which has only been partially loaded. Make sure that all ways connected to this way are present on the map.
+      simple_roundabout: This line cannot be split because this roundabout is part of a larger relation. You must remove it from the relation first.
       connected_to_hidden: This can't be split because it is connected to a hidden feature.
     restriction:
       annotation:

modules/actions/split.js

@@ -428,6 +428,9 @@ export function actionSplit(nodeIds, newWayIds) {
                             }
                         }
                     }
+                    if (way.tags.junction === 'roundabout' && way.isClosed()) {
+                        return 'simple_roundabout';
+                    }
                 }
             }
         }