Add KClass.isFun modifier to reflection

#KT-38881 Fixed

libraries/stdlib/jvm/src/kotlin/reflect/KClass.kt

@@ -127,6 +127,12 @@ public actual interface KClass<T : Any> : KDeclarationContainer, KAnnotatedEleme
     @SinceKotlin("1.1")
     public val isCompanion: Boolean
 
+    /**
+     * `true` if this class is a Kotlin functional interface.
+     */
+    @SinceKotlin("1.4")
+    public val isFun: Boolean
+
     /**
      * Returns `true` if this [KClass] instance represents the same Kotlin class as the class represented by [other].
      * On JVM this means that all of the following conditions are satisfied: