Update map assets references in project.json

Because BASE Editor is reusing binary deserializer for JSON serializer,
the map assets 'source' property in project.json points to binary files
with ".dat" extension. It does not impact on how the editor works
because the asset sources are recreated when importing using UIDs, so
just to be consistent all the .json file references were replaced with
the .dat ones.

project.json

@@ -3,15 +3,15 @@
   "runner": "com.bartlomiejpluta.demo.runner.DemoRunner",
   "maps": [{
     "uid": "d1b85d85-c52a-46f5-b81e-444847f8ddae",
-    "source": "d1b85d85-c52a-46f5-b81e-444847f8ddae.json",
+    "source": "d1b85d85-c52a-46f5-b81e-444847f8ddae.dat",
     "name": "Hero Home"
   }, {
     "uid": "b602601a-e9b0-44bf-bc0d-5f31c9964ba1",
-    "source": "b602601a-e9b0-44bf-bc0d-5f31c9964ba1.json",
+    "source": "b602601a-e9b0-44bf-bc0d-5f31c9964ba1.dat",
     "name": "Hero House"
   }, {
     "uid": "8fbb151f-682a-4357-ba92-157e4097898f",
-    "source": "8fbb151f-682a-4357-ba92-157e4097898f.json",
+    "source": "8fbb151f-682a-4357-ba92-157e4097898f.dat",
     "name": "Forrest"
   }],
   "tileSets": [{