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/promises/then_throw_catch.js

@@ -0,0 +1,6 @@
+var r = null;
+Promise.resolve(1)
+  .then(x => { throw "boom"; })
+  .catch(e => { r = e; });
+__drain();
+r