investigate modding web games

web games have the same .pck any other godot game uses but their loading behaviour might be different.

i assume the most complex part may be getting the files into the correct locations to load from. save files in `user://` are put into the IndexedDB /userfs under the table FILE_DATA. this is probably where we want to load from.

one user in question reportedly managed to inject scripts through a chrome extension.

The web game in question so far was [Rocket Bot Royale](https://www.winterpixel.com/rocketbotroyale), which is on godot 3.5 and also a multiplayer game - we should consider #453