refactor crossing presets to use (new) crossing:markings tag

The tag proposal `crossing:markings` was recently accepted: https://wiki.openstreetmap.org/wiki/Key:crossing:markings

Closes #589
Closes #507
Closes #568
See also #408

data/fields/crossing.json

@@ -1,5 +1,17 @@
 {
     "key": "crossing",
     "type": "combo",
-    "label": "Type"
+    "label": "Type",
+    "options": [
+        "traffic_signals",
+        "uncontrolled",
+        "unmarked"
+    ],
+    "strings": {
+        "options": {
+            "traffic_signals": "Crossing With Traffic Signals",
+            "uncontrolled": "Only Road Markings",
+            "unmarked": "No Road Markings or Traffic Lights"
+        }
+    }
 }

data/fields/crossing/markings.json

@@ -0,0 +1,5 @@
+{
+    "key": "crossing:markings",
+    "type": "combo",
+    "label": "Crossing Markings"
+}