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/format.rs

@@ -122,6 +122,8 @@
 //!   `path_sketch`, `path`) is also first saved in 18.
 //!   A `SurfaceLoft` feature (Surface Loft: `sections`, each a `sketch` with optional `profiles`
 //!   and `curves`) is also first saved in 18.
+//!   A `SurfaceOffset` feature (Offset Surface: `faces` points, optional `body`, `distance`,
+//!   `chain` and `tolerance`) is also first saved in 18.
 
 use serde_json::Value;