feat: fillet radius tapers along an edge to the room there (#749)

Adds `fit: taper` to Fillet: where a constant radius doesn't fit along a straight edge, it follows the room there as a smooth spline, down to zero where a face closes to a point. Taper to Fit is in the Fillet dialog, and the feature's warning says how far it tapered.

crates/doc/src/format.rs

@@ -85,6 +85,8 @@
 //!   edges of a solid or shelled body that become bends). A conversion without `bends` is the
 //!   plate conversion it always was and saves as before; 0.8.0 would drop the bend edges and
 //!   refuse the body as no plate.
+//!   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.
 
 use serde_json::Value;