js-on-sx: format Infinity/-Infinity/NaN per JS spec (+4 String)

js-number-to-string did (str n), which gives OCaml-native "inf"/"-inf"/
"nan"/"-nan" strings. JS spec requires "Infinity"/"-Infinity"/"NaN".

Fix: cond-check js-number-is-nan, and =infinity-value first, fall
through to (str n) for finite.

Unit 521/522, slice 148/148 unchanged.
String scoreboard: 34/100 → 38/100 (+4, S15.5.1.1_A1_T11/T12 family —
String(1/0)/String(-1/0)/String(0/0)).

lib/js/runtime.sx

@@ -1098,7 +1098,15 @@
         (+ i 1)
         (str acc (js-to-string (nth parts i)))))))
 
-(define js-number-to-string (fn (n) (str n)))
+(define
+  js-number-to-string
+  (fn
+    (n)
+    (cond
+      ((js-number-is-nan n) "NaN")
+      ((= n (js-infinity-value)) "Infinity")
+      ((= n (- 0 (js-infinity-value))) "-Infinity")
+      (else (str n)))))
 
 (define
   js-add