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/array_method_chain.js

@@ -0,0 +1 @@
+[5, 2, 8, 1, 4].filter(x => x > 2).map(x => x * 10).reduce((a, b) => a + b, 0)