Add Stringable interface support in Validator.php

[spaze]

• bug fix / new feature? bugfix
• BC break? no

The addRule methods support Stringable for $errorMessage:

	public function addRule(
		callable|string $validator,
		string|Stringable|null $errorMessage = null,
		mixed $arg = null,
	) {

but when I pass an object of class MyRule implements Stringable, I'll get a TypeError:

preg_replace_callback(): Argument #3 ($subject) must be of type array|string, MyRule given

That's because Nette\Forms\Validator only supports Nette\HtmlStringable:
https://github.com/nette/forms/blob/ae11e90098c39576d8744d6bacc35dd858ed4167/src/Forms/Validator.php#L57C33-L58
so an Stringable object will reach preg_replace_callback() and it doesn't know what to do with it.

There are several options here. The first is to return the Stringable object type-casted to string, which is what I've implemented in this PR, another would be to type-cast $message here:

forms/src/Forms/Validator.php

Line 106 in ae11e90

}, $message);

Let me know if I should change the PR to do that.