Implement YieldingTemplate::display*Block()

[nicolas-grekas]

This should make the migration a bit easier (Symfony uses displayBlock() directly in the Form component.)

[GromNaN]

How that would make the migration easier?

The call to displayBlock in Symfony Form is surrounded by ob_start/ob_get_clean. We can fix the calling code in Symfony 5.4+ to check if Twig is using the yielding mode (method_exists($template, 'yieldBlock')) and call it instead.

[nicolas-grekas]

This improves the migration by not breaking ppl that will update twig but not symfony. This will happen during the transition period.

[GromNaN]

If there is a conflict rule in twig for symfony/form < 5.4.36 and the fix is in the new version bugfix, that would prevent issues.

If this PR is merged, it deserves a deprecation warning as this methods are removed from 4.0.

[nicolas-grekas]

twig shouldn't know about symfony/form - that's be entering the package coupling arena
the deprecation is possible but not worth it IMHO

[nicolas-grekas]

Closing in favor of #3999