This is really great. Can't wait for this to be in a stable state, as I am currently struggling to make 3d graphics look good with only flat colors in my game. This and shadow mapping are the only thing missing. Wish we had more people knowledgeable in 3d willing to help out now that its all open source.
I am definitely open to PRs, the main function of frag lighting is outside the addon, the addon is just setting a flag for the hacked vertex shader in the lighting case. Shadow map would also probably be a little outside the addon, and typically requires multiple renders, 1 per light to create shadow map. Something C3 is not really setup for.
Instead would look at baking shadow maps and applying to geometry and use an old technique like this to roughly apply it to moving actors: https://www.reddit.com/r/Unity3D/s/jEOZBCfUSC
Have not tried it myself, but seems plausible.