[php-src] Issue #21428: Add FILTER_VALIDATE_STRLEN for UTF-8 string length validation

Issue: https://github.com/php/php-src/issues/21428
Author: masakielastic

### 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.

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

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