Sketch: infinite construction lines (#1359) (#1547)

Thanks @DuBearte.

crates/doc/src/format/pending/infinite-sketch-lines.rs

@@ -0,0 +1,5 @@
+//! A sketch line can be `infinite` (#1359): construction geometry through its two points, drawn
+//! across the whole view, a boundary for trim and extend along its whole line. Saved only when
+//! set: older designs need no transformation, and a line without it is the line it always was;
+//! 0.13.x (which reads up to 24) asks for an update instead of drawing such a line between its
+//! two points.