Improve VSCode user experience

[maximlt]

This screenshot shows what VS Code displays after I open the parenthesis. It doesn't immediately indicate which engines are available. There are 8 "views" I can explore, some indicating which engine is available and what type will be returned (second screenshot).

Hovering over penguins, I don't see a list of the available engines.

[hoxbro]

I think this is hard (read: I don't know how to do it) because adding these as type hints will make it hard to type-narrow it.

Adding numpy docstrings as suggested #6 will help improve it.