js-on-sx: baseline commit (278/280 unit, 148/148 slice, runner stub)

Initial commit of the lib/js/ tree and plans/ directory. A previous
session left template-string work in progress — 278/280 unit tests pass
(2 failing: tpl part-count off-by-one, escaped-backtick ident lookup).
test262-runner.py and scoreboard are placeholders (0/8 with 7 timeouts);
fixing the runner is the next queue item.

lib/js/test262-scoreboard.json

@@ -0,0 +1,35 @@
+{
+  "totals": {
+    "pass": 0,
+    "fail": 1,
+    "skip": 0,
+    "timeout": 7,
+    "total": 8,
+    "pass_rate": 0.0
+  },
+  "categories": [
+    {
+      "category": "built-ins/Math",
+      "total": 8,
+      "pass": 0,
+      "fail": 1,
+      "skip": 0,
+      "timeout": 7,
+      "pass_rate": 0.0,
+      "top_failures": [
+        [
+          "SyntaxError (parse/unsupported syntax)",
+          1
+        ]
+      ]
+    }
+  ],
+  "top_failure_modes": [
+    [
+      "SyntaxError (parse/unsupported syntax)",
+      1
+    ]
+  ],
+  "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
+  "elapsed_seconds": 40.1
+}