FIC: support fun interfaces in stub builder

idea/testData/decompiler/decompiledText/FunInterfaceDeclaration.expected.kt

@@ -0,0 +1,14 @@
+// IntelliJ API Decompiler stub source generated from a class file
+// Implementation of methods is not available
+
+package test
+
+public final class FunInterfaceDeclaration public constructor() {
+    public fun interface GenericKRunnable<T, R> {
+        public abstract fun invoke(t: T): R
+    }
+
+    public fun interface KRunnable {
+        public abstract fun invoke(): kotlin.Unit
+    }
+}