Add DateIntervalField for editable, localized DateInterval rendering

[Amoifr]

Fix #7374

Storing a DateInterval (Doctrine dateinterval type) in a TextField currently breaks the page with The value of the "..." field can't be converted into a string. The doc itself recommended TextField for this Doctrine type, which doesn't actually work.

This PR introduces a dedicated DateIntervalField, following the plan @javiereguiluz outlined in the issue:

• Forms (new/edit): a single text input expecting an ISO 8601 duration pattern (P1Y2M3D, PT1H30M, etc.). Powered by Symfony's built-in DateIntervalType with widget => single_text and input => dateinterval, so we don't have to maintain a custom transformer.
• Read-only pages (index/detail): a localized, pluralized rendering of the duration (2 years 4 days 6 hours 8 minutes). Each part uses a date_interval.* translation key with the standard Symfony pluralization syntax ({1} %count% year|]1,Inf] %count% years), so locales handle their own grammar.

Per-field opt-out: setFormat('%y years…') falls back to raw DateInterval::format(). Same applies to Crud::setDateIntervalFormat(), which existed already as a stub but was never read by anything.

Other changes in this PR:

• doc/fields.rst: the Doctrine dateinterval row now points to DateIntervalField instead of the (broken) TextField recommendation, and the new field is added to the field list.
• doc/fields/DateIntervalField.rst: new dedicated doc page.
• translations/EasyAdminBundle.{en,fr}.php + 36 other locales: new date_interval.* keys (English placeholder for non-EN/FR per AGENTS.md guidance).
• src/Dto/CrudDto::$dateIntervalFormat: default switched from the (unused) hardcoded English string to null, so the configurator can distinguish "no override → render localized" from "explicit format → raw DateInterval::format()". The public Crud::setDateIntervalFormat(string) signature is unchanged.

Test plan

• tests/Unit/Field/DateIntervalFieldTest.php: 5 new tests cover null value, no-format → sentinel + value preserved, per-field setFormat(), template pluralization (1 year + 2 months + 3 days + 4 hours + 5 minutes + 6 seconds), and zero-duration empty label.
• All tests/Unit/Field/ (610 tests) pass.
• pdo_sqlite was unavailable in my local env, so the existing functional DateIntervalFormatTest (which already only asserts the page renders) wasn't run here, but it still relies on the same Crud::setDateIntervalFormat() API I left untouched.