TextBase support mixed as value

[milous]

• bug fix? no
• new feature? yes
• BC break? no

adds support for

// class without __toString methodfinalclass User {
// ..
}
$repo = function(string$userName): ?User {
returnnewUser;
}
$form = newNette\Forms\Form;
$form->addText('user', 'Username')
->addFilter(function (string$userName) use ($repo) {
returnnewNette\Forms\ScalarValue($userName, $repo($userName)));
} )
;
$form->onSuccess[] = function(Nette\Forms\Form$form, array$values): void {
$values['user'] instanceof User || null;
};