Allow `external` modifier on classes and properties

compiler/frontend/src/org/jetbrains/kotlin/resolve/ModifiersChecker.kt

@@ -84,7 +84,7 @@ object ModifierCheckerCore {
             COROUTINE_KEYWORD to EnumSet.of(VALUE_PARAMETER),
             TAILREC_KEYWORD   to EnumSet.of(FUNCTION),
             SUSPEND_KEYWORD   to EnumSet.of(FUNCTION),
-            EXTERNAL_KEYWORD  to EnumSet.of(FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER),
+            EXTERNAL_KEYWORD  to EnumSet.of(FUNCTION, PROPERTY, PROPERTY_GETTER, PROPERTY_SETTER, CLASS),
             ANNOTATION_KEYWORD to EnumSet.of(ANNOTATION_CLASS),
             CROSSINLINE_KEYWORD to EnumSet.of(VALUE_PARAMETER),
             CONST_KEYWORD     to EnumSet.of(MEMBER_PROPERTY, TOP_LEVEL_PROPERTY),