feat: drag a fillet's radius spline by its handles on the edge (#752)

A spline fillet's radius can now be dragged by handles on the edge, and the spline radius saves under format 17.

crates/doc/src/format.rs

@@ -91,6 +91,9 @@
 //!   edited.
 //!   A fillet's `fit` can be `taper` (the radius follows the room along each edge it doesn't
 //!   fit). Fillets with `exact` or `reduce` save as before; 0.8.0 would refuse the unknown fit.
+//!   A fillet's `type` can be `spline` (the radius along each edge a B-spline: `start`, `knots`,
+//!   and the control radii after `radius` in `radii`). Other fillets save as before; 0.8.0 would
+//!   refuse the unknown type.
 
 use serde_json::Value;