allow direction=forward/backward only on vertices

see https://wiki.openstreetmap.org/w/index.php?title=Key:direction&oldid=2395441#Forward_and_backward

data/fields/direction_vertex.json

@@ -9,6 +9,9 @@
             "both": "Both / All"
         }
     },
+    "geometry": [
+        "vertex"
+    ],
     "autoSuggestions": false,
     "customValues": false
 }

data/presets/traffic_sign/variable_message.json

@@ -1,7 +1,8 @@
 {
     "icon": "temaki-billboard",
     "fields": [
-        "{traffic_sign}"
+        "{traffic_sign}",
+        "direction_vertex"
     ],
     "geometry": [
         "point",