solid.hole: position takes length expressions and follows them (#1678) (#1683)

Thanks @DuBearte.

crates/doc/src/format/pending/hole-position-expressions.rs

@@ -0,0 +1,5 @@
+//! A Hole placed by position can keep that position as length expressions (`at`: `[x, y, z]`,
+//! numbers as numbers, expressions as strings), and is drilled where they evaluate, so it follows
+//! the design's parameters (#1678). `position` still holds the point it was made at. Older designs
+//! need no transformation: a hole without `at` is drilled at `position` as before; 0.13.x asks for
+//! an update instead of drilling a moved hole at its old point.