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/test262-scoreboard.json

@@ -1,37 +1,41 @@
 {
   "totals": {
-    "pass": 77,
-    "fail": 15,
-    "skip": 1,
-    "timeout": 7,
-    "total": 100,
-    "runnable": 99,
-    "pass_rate": 77.8
+    "pass": 14,
+    "fail": 25,
+    "skip": 5,
+    "timeout": 6,
+    "total": 50,
+    "runnable": 45,
+    "pass_rate": 31.1
   },
   "categories": [
     {
-      "category": "built-ins/String",
-      "total": 100,
-      "pass": 77,
-      "fail": 15,
-      "skip": 1,
-      "timeout": 7,
-      "pass_rate": 77.8,
+      "category": "built-ins/Array",
+      "total": 50,
+      "pass": 14,
+      "fail": 25,
+      "skip": 5,
+      "timeout": 6,
+      "pass_rate": 31.1,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          13
+          20
         ],
         [
           "Timeout",
-          7
+          6
+        ],
+        [
+          "TypeError: not a function",
+          2
         ],
         [
           "ReferenceError (undefined symbol)",
-          1
+          2
         ],
         [
-          "SyntaxError (parse/unsupported syntax)",
+          "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
           1
         ]
       ]
@@ -40,22 +44,26 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      13
+      20
     ],
     [
       "Timeout",
-      7
+      6
+    ],
+    [
+      "TypeError: not a function",
+      2
     ],
     [
       "ReferenceError (undefined symbol)",
-      1
+      2
     ],
     [
-      "SyntaxError (parse/unsupported syntax)",
+      "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
       1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 486.8,
+  "elapsed_seconds": 132.0,
   "workers": 1
 }