js-on-sx: top-level this resolves to the global object

Per ES non-strict script semantics, top-level this is the global
object (window/global/globalThis). Was throwing "Undefined symbol:
this". Two-part fix:
1. js-global-this runtime variable set to js-global after globals
   are defined; js-this falls back to it when no this is active.
2. js-eval wraps transpiled body in (let ((this (js-this))) ...)
   so JS this resolves to bound this, or top-level to global.
Fixes String(this), this.Object === Object, etc.
built-ins/Object: 46/50 → 47/50. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,37 +1,53 @@
 {
   "totals": {
-    "pass": 46,
-    "fail": 4,
-    "skip": 0,
-    "timeout": 0,
-    "total": 50,
-    "runnable": 50,
-    "pass_rate": 92.0
+    "pass": 86,
+    "fail": 11,
+    "skip": 1,
+    "timeout": 2,
+    "total": 100,
+    "runnable": 99,
+    "pass_rate": 86.9
   },
   "categories": [
     {
-      "category": "built-ins/Object",
-      "total": 50,
-      "pass": 46,
-      "fail": 4,
-      "skip": 0,
-      "timeout": 0,
-      "pass_rate": 92.0,
+      "category": "built-ins/String",
+      "total": 100,
+      "pass": 86,
+      "fail": 11,
+      "skip": 1,
+      "timeout": 2,
+      "pass_rate": 86.9,
       "top_failures": [
         [
-          "ReferenceError (undefined symbol)",
-          4
+          "Test262Error (assertion failed)",
+          10
+        ],
+        [
+          "Timeout",
+          2
+        ],
+        [
+          "SyntaxError (parse/unsupported syntax)",
+          1
         ]
       ]
     }
   ],
   "top_failure_modes": [
     [
-      "ReferenceError (undefined symbol)",
-      4
+      "Test262Error (assertion failed)",
+      10
+    ],
+    [
+      "Timeout",
+      2
+    ],
+    [
+      "SyntaxError (parse/unsupported syntax)",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 64.1,
+  "elapsed_seconds": 179.5,
   "workers": 1
 }