[TwigBridge][TwigBundle] Add current locale to AppVariable

[SVillette]

Q	A
Branch?	6.3
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	Fix #49870
License	MIT
Doc PR	symfony/symfony-docs#18190

As stated in #49870, they were no way to get the current locale without passing it through a variable when rendering a template within LocaleSwitcher::runWithLocale().

#[AsController]
final class HomeController
{
    #[Route('/', name: 'app_home')]
    public function __invoke(LocaleSwitcher $localeSwitcher, Environment $twig): Response
    {
        $localeSwitcher->setLocale('en');

        return $localeSwitcher->runWithLocale('fr', function () use ($twig) {
            return new Response($twig->render('index.html.twig'));
        });
    }
}

{{ app.locale }} // fr

A doc PR will be submitted if this change is accepted.

[SVillette]

About the failing tests, I may need some help / directions here.

• The Integration / Integration (8.1) seems to be unrelated to this PR (Cache related).
• The Unit Tests / Unit Tests (8.1) is failing because AppVariable::setLocaleSwitcher() does not have a return type and is not listed in .github/expected-missing-return-types.diff. Updating this file should resolve the issue but I can't do it as they are bunch of other changes here.
• The Unit Tests / Unit Tests (8.1, low-deps) fails because LocaleSwitcher class is missing. Do I have to skip the test if the class does not exist ?
• Finally, continuous-integration/appveyor/pr fail seems unrelated to this PR (VarDumper issue).

[stof]

For low-deps, you need to increase the lower bound of the requirement

[SVillette]

Thank you for the input, CI is still failling but not because of this PR.

[fabpot]

Thank you @SVillette.

[curry684]

When installing the Twig Bundle 6.3.0-BETA1 this breaks the application if you didn't directly add the bridge as well:

!!    [Symfony\Component\ErrorHandler\Error\UndefinedMethodError]
!!    Attempted to call an undefined method named "setLocaleSwitcher" of class "S
!!    ymfony\Bridge\Twig\AppVariable".

Bundle 6.3 is thus incompatible with Bridge 6.2 yet this is not enforced through dependencies.