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/class_basic.js

@@ -0,0 +1,6 @@
+class Point {
+  constructor(x, y) { this.x = x; this.y = y; }
+  sum() { return this.x + this.y; }
+}
+var p = new Point(3, 4);
+p.sum()