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

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

@@ -0,0 +1,11 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+package foo
+
+@JsModule("./externalFunction.mjs")
+external fun foo(y: Int): Int = definedExternally
+
+fun box(): String {
+    assertEquals(65, foo(42))
+    return "OK"
+}