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_throws_error_object.js

@@ -0,0 +1,9 @@
+async function failing() { throw new Error("bad"); }
+async function main() {
+  try { await failing(); return "unreachable"; }
+  catch (e) { return e.message; }
+}
+var r = null;
+main().then(v => { r = v; });
+__drain();
+r