[JS IR] Export nested objects

Companion objects are exported as ParentClass.Companion.
Companion object's members are not exposed to its parent class —
one must reference the companion object explicitly if they want to
access its members.

#KT-43783 Fixed

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

@@ -0,0 +1,58 @@
+// EXPECTED_REACHABLE_NODES: 1265
+// RUN_PLAIN_BOX_FUNCTION
+// INFER_MAIN_MODULE
+// SKIP_MINIFICATION
+// SKIP_DCE_DRIVEN
+// SKIP_NODE_JS
+
+// See KT-43783
+
+// MODULE: nestedObjectExport
+// FILE: lib.kt
+
+@JsExport
+class Abc {
+    companion object AbcCompanion {
+        fun xyz(): String = "Companion object method OK"
+
+        val prop: String
+            get() = "Companion object property OK"
+    }
+}
+
+@JsExport
+class Foo {
+    companion object {
+        fun xyz(): String = "Companion object method OK"
+
+        val prop: String
+            get() = "Companion object property OK"
+    }
+}
+
+@JsExport
+sealed class MyEnum(val name: String) {
+    object A: MyEnum("A")
+    object B: MyEnum("B")
+    object C: MyEnum("C")
+}
+
+// FILE: test.js
+
+function box() {
+    const abcCompanion = nestedObjectExport.Abc.AbcCompanion;
+
+    if (abcCompanion.xyz() != 'Companion object method OK') return 'companion object function failure';
+    if (abcCompanion.prop != 'Companion object property OK') return 'companion object property failure';
+
+    const justCompanion = nestedObjectExport.Foo.Companion;
+
+    if (justCompanion.xyz() != 'Companion object method OK') return 'companion object function failure';
+    if (justCompanion.prop != 'Companion object property OK') return 'companion object property failure';
+
+    if (nestedObjectExport.MyEnum.A.name != 'A') return 'MyEnum.A failure';
+    if (nestedObjectExport.MyEnum.B.name != 'B') return 'MyEnum.B failure';
+    if (nestedObjectExport.MyEnum.C.name != 'C') return 'MyEnum.C failure';
+
+    return 'OK';
+}