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/async_nested_calls.js

@@ -0,0 +1,7 @@
+async function inner(x) { return x * 2; }
+async function middle(x) { var r = await inner(x); return r + 1; }
+async function outer(x) { var r = await middle(x); return r * 3; }
+var result = null;
+outer(5).then(v => { result = v; });
+__drain();
+result