js-on-sx: delete operator

js-transpile-unop intercepts 'delete' before transpiling the
operand. Maps to (js-delete-prop obj key) for members and indexed
access. Runtime js-delete-prop sets the dict value to js-undefined
and returns true.

444/446 unit (+2), 148/148 slice unchanged.

lib/js/runtime.sx

@@ -1825,6 +1825,15 @@
 
 (define Object {:entries js-object-entries :values js-object-values :freeze js-object-freeze :assign js-object-assign :keys js-object-keys})
 
+(define
+  js-delete-prop
+  (fn
+    (obj key)
+    (cond
+      ((dict? obj)
+        (begin (dict-set! obj (js-to-string key) js-undefined) true))
+      (else true))))
+
 (define
   js-optchain-get
   (fn