js-on-sx: arrays accept numeric-string property keys

JS arrays must treat string indices that look like numbers ("0",
"42") as the corresponding integer slot. js-get-prop and js-list-set!
only handled numeric key, falling through to undefined / no-op for
string keys. Added a (and (string-typed key) (numeric? key)) clause
that converts via js-string-to-number and recurses with the integer
key. built-ins/Array: 13/45 → 14/45. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2639,6 +2639,13 @@
                 (and (>= key 0) (< key (len obj)))
                 (nth obj (js-num-to-int key))
                 js-undefined))
+            ((and (= (type-of key) "string") (js-is-numeric-string? key))
+              (let
+                ((idx (js-num-to-int (js-string-to-number key))))
+                (if
+                  (and (>= idx 0) (< idx (len obj)))
+                  (nth obj idx)
+                  js-undefined)))
             ((= key "push") (js-array-method obj "push"))
             ((= key "pop") (js-array-method obj "pop"))
             ((= key "shift") (js-array-method obj "shift"))
@@ -2791,6 +2798,8 @@
               ((< i n) (set-nth! lst i val))
               ((= i n) (append! lst val))
               (else (do (js-pad-list! lst n i) (append! lst val))))))
+        ((and (= (type-of key) "string") (js-is-numeric-string? key))
+          (js-list-set! lst (js-string-to-number key) val))
         ((= key "length") nil)
         (else nil))))
   (define