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

libraries/stdlib/js-v1/src/kotlin/currentBeMisc.kt

@@ -9,4 +9,6 @@ package kotlin.js
 // Used for js.translator/testData/box/number/mulInt32.kt
 @library
 @JsName("imulEmulated")
-internal fun imul(x: Int, y: Int): Int = definedExternally
+internal fun imul(x: Int, y: Int): Int = definedExternally
+
+internal inline fun isArrayish(o: dynamic) = js("Kotlin").isArrayish(o)