Offset Surface: faces copied out at a distance, exact or fitted (#758) (#872)

Offsets picked faces along their normals into a new surface body: planes and cylinders move exactly, other faces get a fitted B-spline within tolerance, and offsets that would fold over are refused. Adds the SurfaceOffset feature (noted under format 18), the surface.offset engine command and an Offset Surface dialog.

Closes #758.

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>

crates/doc/src/clipboard.rs

@@ -305,6 +305,7 @@ impl FeatureKind {
             | K::SplitFace { body, .. }
             | K::OffsetFace { body, .. }
             | K::DeleteFace { body, .. }
+            | K::SurfaceOffset { body, .. }
             | K::ReplaceFace { body, .. }
             | K::SheetFlange { body, .. }
             | K::SheetHem { body, .. }