js-on-sx: js-num-from-string uses pow (float) for exponent

js-pow-int 10 20 overflows int64 (10^20 > 2^63), so numeric literals
like 1e20 and 100000000000000000000 were parsing as
-1457092405402533888. The pow primitive uses float-domain
exponentiation and produces 1e+20 correctly. Single call swap in
js-num-from-string. built-ins/String (with --restart-every 1):
67/99 → 70/99. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1073,7 +1073,8 @@
       ((trimmed (js-trim s)))
       (cond
         ((= trimmed "") 0)
-        ((js-hex-prefix? trimmed) (js-parse-hex trimmed 2 0))
+        ((js-hex-prefix? trimmed)
+          (js-parse-hex trimmed 2 0))
         (else
           (let
             ((esplit (js-find-exp-char trimmed)))
@@ -1082,12 +1083,28 @@
               (let
                 ((mant (js-string-slice trimmed 0 esplit))
                   (expstr
-                    (js-string-slice trimmed (+ esplit 1) (len trimmed))))
+                    (js-string-slice
+                      trimmed
+                      (+ esplit 1)
+                      (len trimmed))))
                 (let
                   ((m (js-parse-decimal mant 0 0 1 false 0))
-                    (e (js-parse-decimal expstr 0 0 1 false 0)))
-                  (* m (js-pow-int 10 e))))
-              (js-parse-decimal trimmed 0 0 1 false 0))))))))
+                    (e
+                      (js-parse-decimal
+                        expstr
+                        0
+                        0
+                        1
+                        false
+                        0)))
+                  (* m (pow 10 e))))
+              (js-parse-decimal
+                trimmed
+                0
+                0
+                1
+                false
+                0))))))))
 
 (define js-trim (fn (s) (js-trim-left (js-trim-right s))))