builtin: add none str() (fix #12964) (#12967)

vlib/builtin/option.v

@@ -87,3 +87,7 @@ pub fn (e &Error) free() {
 pub fn (n &None__) free() {
 	unsafe { n.msg.free() }
 }
+
+pub fn (_ none) str() string {
+	return 'none'
+}

vlib/v/tests/string_optional_none_test.v

@@ -0,0 +1,14 @@
+struct MyError {
+	code int
+	msg  string
+}
+
+fn foo() int | none | IError {
+	return IError(MyError{})
+}
+
+fn test_string_optional_none() {
+	x := foo()
+	println(x)
+	assert true
+}