Adds downfall environment

[edbeeching]

Remaining todos:

• add onnx model
• add video
• add hyperparameters for training (default with 2M steps with 4 parallel envs)
• add example to hub and to tests on the godot-rl main repo Adds downfall env to tests and hub godot_rl_agents#176

[Ivan-267]

Nicely made.

I've made a quick test (not full training), it seems to be working OK and is starting to learn.

I'll add some comments just as notes, the env is working fine as is so no alterations are necessary.

[Ivan-267]

I just retried with the onnx and got an inference error.
Solved by renaming the .csproj and .sln files to the name of the project.

It looks good and the agents are able to solve the levels.

[edbeeching]

@Ivan-267 ah thanks. Yes, I renamed these locally. I need to push the new files.