Add FILTER_VALIDATE_STRLEN for UTF-8 string length validation

Description

I would like to propose a new filter validator FILTER_VALIDATE_STRLEN that validates the length of a UTF-8 string.

The length is counted in Unicode code points. Grapheme clusters are not considered.

The validator supports min_len and max_len options and does not depend on the mbstring extension.

A prototype implementation is provided in the accompanying pull request.

$options = [
    'options' => [
        'max_len' => 2,
        'default' => 'error',
    ],
];

var_dump(
    filter_var('🐘🐘🐘', FILTER_VALIDATE_STRLEN, $options)
);

Commencez par examiner l’implémentation du prototype fournie et les points d’entrée existants du validateur de filtres. Vérifiez les exigences concernant min_len, max_len, default, le comptage des points de code UTF-8 et no-mbstring, puis vérifiez que le validateur gère l’exemple et ses cas limites de manière cohérente.