Register the missing extra callables in MissingExtensionSuggestor

[smnandre]

Extensions.php mirrors what each twig/*-extra package declares, so MissingExtensionSuggestor can answer an unknown name with try running "composer require twig/string-extra" rather than a bare Unknown "slug" filter.

This PR adds the twelve callables missing from it:

• html-extra: html_attr_merge and html_attr_type filters, html_cva and html_attr functions
• intl-extra: format_list filter, language_names, script_names, country_names, locale_names, currency_names and timezone_names functions
• string-extra: slug, plural and singular filters

ExtensionsTest checks the catalog both ways:

• every listed name resolves on an Environment holding the extension,
• every declared filter, function and tag is listed

[fabpot]

Thank you @smnandre.