js-on-sx: Boolean(NaN) === false

js-to-boolean was returning true for NaN because NaN != 0 by IEEE
semantics — the (= v 0) test fell through to the truthy else.
Per ES, NaN is one of the falsy values. Added a
(js-number-is-nan v) clause.
built-ins/Boolean: 19/27 → 21/27. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1007,6 +1007,7 @@
       ((= v nil) false)
       ((= v false) false)
       ((= v 0) false)
+      ((and (= (type-of v) "number") (js-number-is-nan v)) false)
       ((= v "") false)
       (else true))))