Special case multipolygon preset icon

Multipolygons have geometry === "area", so normally
wouldn't trigger a 'preset-icon-relation' class.

This preset usually matches only in the case where
the tags are on the outer way and the multipolygon
itself is untagged. Otherwise, a more specific area
preset will match.

data/presets/presets.json

@@ -4489,6 +4489,7 @@
     },
     "type/multipolygon": {
         "geometry": [
+            "area",
             "relation"
         ],
         "tags": {

data/presets/presets/type/multipolygon.json

@@ -1,5 +1,6 @@
 {
     "geometry": [
+        "area",
         "relation"
     ],
     "tags": {