assets.py: output a full JS file that can be sourced directly

scripts/assets.py

@@ -304,10 +304,12 @@ def stitch_sprites(metadata, filename_base):
   image_filename = os.path.expanduser(filename_base) + '.png'
   print('saving image to', image_filename)
   pygame.image.save(output, image_filename)
-  json_filename = os.path.expanduser(filename_base) + '.json'
+  json_filename = os.path.expanduser(filename_base) + '.js'
   print('saving json to', json_filename)
   with open(json_filename, 'w') as json_file:
+    json_file.write(' const spritesheet_json = ')
     json.dump(output_json, json_file, sort_keys=True, indent=2)
+    json_file.write(';')
 
 
 def main(args):