Get CrossSection from Manifold and plane

[harmanpa]

There doesn't appear to be a way of creating a CrossSection from a Manifold. This would be a useful feature for creating a 2D slice from a 3D mesh.

[pca006132]

Yes we did think about this in #127, but it seems that we forgot it and no one remember to implement it.

[elalish]

Agreed, I've been wanting this, but haven't gotten around to implementing it. Shouldn't be too hard; we can use a similar API to trimByPlane.

[elalish]

Any interest in contributing @harmanpa?

[johnnybigert]

Plus one on this one, would be very useful to have :)

[harmanpa]

Awesome, thank you @elalish