Add slice primitive

[sorki]

I've started to implement projection similar to OpenSCAD but got stuck on its base case which is a projection of all points onto a plane, while the cut = true case is pretty simple.

While reading Ruckus docs I've realized we can have this implemented as slice primitive instead (naming hard..) and even stumbled on MAYBEFIXME suggesting exactly that.

This probably doesn't belong to 2D or 3D prims but a shared one as outlined in #446. Edit: think it's fine for now since extrude is in 3D prims.

Also needs

• [x] Tests
• [x] Example
• [x] Changelog entry
• [x] SymbolicFormats output (while it doesn't make much sense to add it to ExtOpenSCAD as it is a special case of projection, we can maybe generate projection(cut = true) (at least for 3D to 2D)

[sorki]

Will add changelog entries after #473 is merged