[JS BE] Extract backend specific code from shared stdlib sources

libraries/stdlib/js/src/kotlin/collections/ArraysJs.kt

@@ -14,7 +14,7 @@ internal fun <T> Array<out T>.contentDeepHashCodeImpl(): Int {
     for (element in this) {
         val elementHash = when {
             element == null -> 0
-            js("Kotlin").isArrayish(element) -> (element.unsafeCast<Array<*>>()).contentDeepHashCodeImpl()
+            isArrayish(element) -> (element.unsafeCast<Array<*>>()).contentDeepHashCodeImpl()
 
             element is UByteArray   -> element.contentHashCode()
             element is UShortArray  -> element.contentHashCode()

libraries/stdlib/js/src/kotlin/dynamic.kt

@@ -38,7 +38,7 @@ public operator fun dynamic.iterator(): Iterator<dynamic> {
     return when {
         this["iterator"] != null ->
             this["iterator"]()
-        js("Kotlin").isArrayish(r) ->
+        isArrayish(r) ->
             r.unsafeCast<Array<*>>().iterator()
 
         else ->