[String] Add singularize and pluralize filter

[welcoMattic]

This PR follows up this one in Symfony symfony/symfony#57245.

It adds 2 new filters singularize and pluralize. It uses the String component, especially EnglishInflector and FrenchInflector.

Examples:

{{ 'person'|pluralize('en') }} // persons
{{ 'person'|pluralize('en', false) }} // ['persons', 'people']


{{ 'personne'|pluralize('fr') }} // personnes
{{ 'personne'|pluralize('fr', false) }} // ['personnes']


{{ 'persons'|singluarize('en') }} // person
{{ 'people'|singluarize('en') }} // person
{{ 'persons'|singluarize('en', false) }} // ['person']

{{ 'personnes'|singluarize('fr') }} // personne
{{ 'personnes'|singluarize('fr', false) }} // ['personne']



{{ 'persona'|pluralize('it') }} // \InvalidArgumentException('Language "it" is not supported.')
{{ 'persone'|singluarize('it') }} // \InvalidArgumentException('Language "it" is not supported.')

[fabpot]

I've just pushed an additional commit that makes some significant changes. It contains many tweaks for the docs and the code, but most importantly, it renames the filters to singular and plural. Tell me what you think about these changes.

[GromNaN]

If that can motivate someone to implement pluralization in Italian, just to break this test 😂

[fabpot]

Thank you @welcoMattic.