Add JS specific proto comparison test

Original commit: 239943867b

jps/jps-plugin/testData/comparison/jsOnly/externals/old.kt

@@ -0,0 +1,15 @@
+package test
+
+external class ExternalClass {
+    fun removedExternalFun()
+    val removedExternalVal: Int
+}
+
+class ClassBecameExternal
+external class ClassBecameNonExternal
+
+external fun removedExternalFun()
+external val removedExternalVal: Int
+
+fun funBecameExternal() {}
+external fun funBecameNonExternal()