feat(modify): TXTEXP turns text into its character outlines

TXTEXP replaces each selected TEXT and MTEXT with one polyline per
glyph contour, on the text's layer and with its properties: a
lightweight polyline for flat text, a 3D polyline for tilted text.
The outlines come from the same glyph tessellation the text renders
with, so stroke fonts give their strokes and outline fonts their
closed contours.

Refs D#1665

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/draw/select.rs

@@ -360,6 +360,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "MOVE",
         "ROTATE",
         "SCALE",
-        "STRETCH"
+        "STRETCH",
+        "TXTEXP"
     ]
 }); // SelectObjectsCommand