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-slice/objects/counter_closure.js

@@ -0,0 +1,10 @@
+function makeCounter() {
+  var n = 0;
+  return {
+    inc: function() { n = n + 1; return n; },
+    val: function() { return n; }
+  };
+}
+var c = makeCounter();
+c.inc(); c.inc(); c.inc();
+c.val()