FIC: introduce language feature, modifiers checks and basic tests

compiler/testData/diagnostics/tests/funInterface/basicFunInterface.txt

@@ -0,0 +1,8 @@
+package
+
+public interface Foo {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}