Site Health: Add Intl to the list of recommended PHP extensions.

**Why is the PHP Intl extension important?**

WordPress is a global, international software, with support for a multitude of languages and with infinite combinations. Approximately half of the installations are in a language that is not the default (English), and this leads us to think about localization, transliteration, encoding conversions, calendar operations, collation… in short, everything you have with the different languages and formats that are around the planet. And this is what the [https://www.php.net/manual/en/intro.intl.php PHP Intl extension] provides.

**What do we as the WordPress Community gain from this extension?**

This extension provides a lot of functions for better internationalization support, including but not limited to:
* [https://www.php.net/manual/en/collator.compare.php collator_compare()] to compare Unicode text strings
* [https://www.php.net/manual/en/numberformatter.format.php numfmt_format()] to format a number according to the selected locale
* the [https://www.php.net/manual/en/normalizer.normalize.php normalization] of characters
* the [https://www.php.net/manual/en/messageformatter.formatmessage.php formatting] of messages
* getting the [https://www.php.net/manual/en/intlcalendar.getfirstdayofweek.php first day of the week] according to the locale.


In addition to functionality and ease of development, the extension can also help improve security, with classes like `Spoofchecker` that can tell you [https://www.php.net/manual/en/spoofchecker.areconfusable.php if ‘google.com’, ‘goog1e.com’ can confuse the user], or functions related to Internet domains, both to convert an [https://www.php.net/manual/en/function.idn-to-ascii.php IDN domain to text] and [https://www.php.net/manual/en/function.idn-to-utf8.php text to IDN].

**Hosting Team Recommendation**

Taking into account that WordPress continues to grow, the Hosting Team has considered a good recommendation, but not an obligation, for all hosts that work with WordPress to offer this extension, by default, to all users.

Reference: [https://make.wordpress.org/hosting/2021/05/20/why-hosters-should-install-the-php-intl-extension/ Why hosters should install the PHP-intl extension].

Follow-up to [44986], [46268].

Props zodiac1978, JavierCasares, jrf, Clorith, josklever.
Fixes #52654.

git-svn-id: https://develop.svn.wordpress.org/trunk@51804 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-site-health.php

@@ -941,6 +941,10 @@ public function get_test_php_extensions() {
 				'function' => 'iconv',
 				'required' => false,
 			),
+			'intl'      => array(
+				'extension' => 'intl',
+				'required'  => false,
+			),
 			'mcrypt'    => array(
 				'extension'    => 'mcrypt',
 				'required'     => false,