Add star model

[hannobraun]

I wanted to add an example model with a star shape, both to test/demonstrate support for concave sketches and because I think the code that generates the star shape serves as a nice demonstration for the kind of thing that Code-CAD is useful for.

Results are decidedly mixed 😄

I haven't had a chance to look into it yet in any detail, although I'm pretty sure this displays two distinct problems:

• Not all triangles are generated counter-clockwise.
• Some triangles are discarded erroneously.

I'll look into it when I get a chance and will open/fix issues as required.

[hannobraun]

I've opened #59 and #60. Already fixed #59 in #61. Here's what the model looks like now:

Slightly better. Still working on #60.

[hannobraun]

Looks correct now, after #81.

This is not merge-ready though. I still want to put a star-shaped hole into it.

[hannobraun]

And done!