fix(KT-46961): add support of nested external enums.

js/js.translator/testData/box/native/externalNestedEnum.js

@@ -0,0 +1,6 @@
+var O = {
+    Foo: {
+        A: {},
+        B: {}
+    }
+};

js/js.translator/testData/box/native/externalNestedEnum.kt

@@ -0,0 +1,24 @@
+// KT-46961
+// IGNORE_BACKEND: JS
+
+external object O {
+    enum class Foo {
+        A, B
+    }
+}
+
+fun box(): String {
+    val a = foo(O.Foo.A)
+    val b = foo(O.Foo.B)
+
+    if (a != "!") return "fail1: $a"
+    if (b != "@") return "fail2: $b"
+
+    return "OK"
+}
+
+fun foo(x: O.Foo) = when (x) {
+    O.Foo.A -> "!"
+    O.Foo.B -> "@"
+    else -> ""
+}