Add aircraft holding position type field (close #100)

interim/source_strings.yaml

@@ -1145,6 +1145,16 @@ en:
       historic/wreck/visible_at_low_tide:
         # 'wreck:visible_at_low_tide=*'
         label: Visible At Low Tide
+      holding_position/type:
+        # 'holding_position:type=*'
+        label: Type
+        options:
+          # 'holding_position:type=ILS'
+          ILS: Instrument Landing System
+          # 'holding_position:type=intermediate'
+          intermediate: Taxiway-Taxiway Intersection
+          # 'holding_position:type=runway'
+          runway: Runway
       hoops:
         # hoops=*
         label: Hoops