Sheet metal: sheet.perforate, a perforation field exported to the flat pattern and DXF (#732) (#1041)

Adds sheet.perforate: round holes on a staggered or square grid over the flat sheet, kept clear of edges, cut-outs and bend zones. The holes go to the flat pattern, the DXF and the measured volume rather than the B-rep, so a panel can carry many thousands of them.

crates/doc/src/format.rs

@@ -143,6 +143,10 @@
 //!   no transformation, and an extrude without `thin` saves as before; the version stamp makes
 //!   0.10.0 (which reads up to 19) ask for an update instead of ignoring the wall and filling
 //!   the sketch's profiles.
+//!   A `sheet_perforate` feature (Perforate: `diameter`, `pitch`, optional `margin`,
+//!   `pattern` and `body`): a perforation field stored on a sheet, for the flat pattern, the DXF
+//!   and the measure. It is also first saved in 20; 0.10.0 asks for an update instead of
+//!   meeting a feature it can't read.
 
 use serde_json::Value;