Add external keyword support for UL

Fixed #KT-34923

compiler/testData/asJava/ultraLightClasses/simpleFunctions.kt

@@ -9,5 +9,7 @@ class Foo {
   operator fun plus(increment: Int): Foo {}
   fun String.onString(a: (Int) -> Any?): Foo {}
 
+  external fun externalFun(a: Int): String { return "" }
+
   class Inner {}
 }

compiler/testData/asJava/ultraLightFacades/simpleFunctions.kt

@@ -7,3 +7,5 @@ public fun nullableVararg(vararg o: Any?): Unit
 
 operator fun plus(increment: Int): Foo {}
 fun String.onString(a: (Int) -> Any?): Foo {}
+
+external fun externalFun(a: Int): String { return "" }