JS IR: per-module .js generation support

js/js.translator/testData/box/crossModuleRefIR/export.kt

@@ -0,0 +1,32 @@
+// IGNORE_BACKEND: JS
+// SPLIT_PER_MODULE
+// RUN_PLAIN_BOX_FUNCTION
+// EXPECTED_REACHABLE_NODES: 1316
+
+// MODULE: lib1
+// FILE: lib1.kt
+
+@JsExport
+fun O(): String = "O"
+
+// MODULE: lib2(lib1)
+// FILE: lib2.kt
+
+@JsExport
+fun K(): String = "K"
+
+
+// MODULE: main(lib1, lib2)
+// FILE: main.kt
+
+@JsExport
+fun test() = O() + K()
+
+// FILE: test.js
+
+function box() {
+
+    if (main.test() != "OK") return "fail 1";
+
+    return kotlin_lib1.O() + kotlin_lib2.K();
+}