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/async/await_basic.js

@@ -0,0 +1,9 @@
+async function add(a, b) { return a + b; }
+async function main() {
+  var r = await add(3, 4);
+  return r * 10;
+}
+var result = null;
+main().then(v => { result = v; });
+__drain();
+result