The shape types are set as “constants” here in script layer of the physics code:
This means that if you iterate _G.physics at runtime you should see them together with all the other functions and constants.
But that is not enough for the editor to know about them, or for the API reference to include them. The code must also have code annotations that we can pick up and use when we generate the API docs and auto-complete for the editor, like these ones for the joint types:
Would you like to attempt a PR to add the shape type constants?