mapgen: make add_mapgen_update_func work with mapgen_update_ids

if the Json object has a mapgen_update_id string, use that instead of
parsing inline mapgen_update.

doc/MISSIONS_JSON.md

@@ -160,7 +160,7 @@ The `update_mapgen`` object or array provides a way to modify existing overmap t
 
 As an array, `update_mapgen` consists of two or more `update_mapgen` objects.
 
-As an object, `update_mapgen` contains any valid JSON mapgen objects.  The objects are placed on the mission target terrain from "assign_mission_target" or optionally the closest overmap terrain specified by the `om_terrain` and `om_special` fields.
+As an object, `update_mapgen` contains any valid JSON mapgen objects.  The objects are placed on the mission target terrain from "assign_mission_target" or optionally the closest overmap terrain specified by the `om_terrain` and `om_special` fields.  If "mapgen_update_id" is specified, the "mapge_update" object with the matching "mapgen_update_id" will be executed.
 
 See doc/MAPGEN.md for more details on JSON mapgen and `update_mapgen`.
 

src/mapgen.cpp

@@ -8669,6 +8669,15 @@ bool update_mapgen_function_json::update_map( const tripoint &omt_pos, int offse
 
 mapgen_update_func add_mapgen_update_func( JsonObject &jo, bool &defer )
 {
+    if( jo.has_string( "mapgen_update_id" ) ) {
+        const std::string mapgen_update_id = jo.get_string( "mapgen_update_id" );
+        const auto update_function = [mapgen_update_id]( const tripoint & omt_pos,
+        mission * miss ) {
+            run_mapgen_update_func( mapgen_update_id, omt_pos, miss, false );
+        };
+        return update_function;
+    }
+
     update_mapgen_function_json json_data( "" );
     mapgen_defer::defer = defer;
     if( !json_data.setup_update( jo ) ) {