[JS IR] Support per-file mode and ES modules

js/js.translator/testData/box/esModules/jsModule/topLevelVarargFun.kt

@@ -0,0 +1,15 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+@JsModule("./topLevelVarargFun.mjs")
+external fun foo(vararg arg: String): String
+
+fun box(): String {
+    val x = arrayOf("a", "b")
+    var r = foo(*x)
+    if (r != "(ab)") return "fail1: $r"
+
+    r = foo("c", "d")
+    if (r != "(cd)") return "fail2: $r"
+
+    return "OK"
+}