js-on-sx: scope var defines + js-args for call args

JS top-level var was emitting (define <name> X) at SX top level,
permanently rebinding any SX primitive of that name (e.g. var list
= X broke (list ...) globally). Two-part fix:
1. wrap transpiled program in (let () ...) in js-eval so defines
   scope to the eval and don't leak.
2. rename call-args constructor in js-transpile-args from list to
   js-args (a variadic alias) so even within the eval's own scope,
   JS vars named list don't shadow arg construction.
Array-literal transpile keeps list (arrays must be mutable).
built-ins/Object: 41/50 → 42/50. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1153,6 +1153,8 @@
                 false
                 0))))))))
 
+(define js-args (fn (&rest args) args))
+
 (define js-trim (fn (s) (js-trim-left (js-trim-right s))))
 
 (define