Define %Temporal.Foo.prototype% intrinsics

These intrinsics are defined in the spec, but were not yet needed in the
polyfill, so hadn't been defined. They will be needed in order for the
ECMA-402 calendars to define custom calendar properties on
%Temporal.Date.prototype%, for example.

polyfill/lib/intrinsicclass.mjs

@@ -74,6 +74,7 @@ export function MakeIntrinsicClass(Class, name) {
   }
 
   DefineIntrinsic(name, Class);
+  DefineIntrinsic(`${name}.prototype`, Class.prototype);
 }
 
 export function DefineIntrinsic(name, value) {