FIC: introduce language feature, modifiers checks and basic tests

compiler/testData/codegen/box/inlineClasses/funInterface/basicFunInterface.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +FunctionInterfaceConversion
+
+fun interface Foo {
+    fun invoke(): String
+}
+
+class A : Foo {
+    override fun invoke(): String {
+        return "OK"
+    }
+}
+
+fun box(): String {
+    return A().invoke()
+}