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

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

@@ -0,0 +1,16 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+package foo
+
+@JsModule("./externalObject.mjs")
+external object A {
+    val x: Int = definedExternally
+
+    fun foo(y: Int): Int = definedExternally
+}
+
+fun box(): String {
+    assertEquals(23, A.x)
+    assertEquals(65, A.foo(42))
+    return "OK"
+}