js-on-sx: top-level this resolves to the global object

Per ES non-strict script semantics, top-level this is the global
object (window/global/globalThis). Was throwing "Undefined symbol:
this". Two-part fix:
1. js-global-this runtime variable set to js-global after globals
   are defined; js-this falls back to it when no this is active.
2. js-eval wraps transpiled body in (let ((this (js-this))) ...)
   so JS this resolves to bound this, or top-level to global.
Fixes String(this), this.Object === Object, etc.
built-ins/Object: 46/50 → 47/50. conformance.sh: 148/148.

lib/js/transpile.sx

@@ -1498,7 +1498,7 @@
   (fn
     (src)
     (let
-      ((result (eval-expr (list (quote let) (list) (js-transpile (js-parse (js-tokenize src)))))))
+      ((result (eval-expr (list (quote let) (list (list (js-sym "this") (list (js-sym "js-this")))) (js-transpile (js-parse (js-tokenize src)))))))
       (js-drain-microtasks!)
       result)))