feature symfony#24200 Added an alias for FlashBagInterface in config (tifabien)

This PR was submitted for the 3.3 branch but it was merged into the 3.4 branch instead (closes symfony#24200).

Discussion
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Added an alias for FlashBagInterface in config

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
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b6b6d31 Added an alias for FlashBagInterface in config

Author: nicolas-grekas

src/Symfony/Bundle/FrameworkBundle/Resources/config/session.xml

@@ -38,6 +38,7 @@
         </service>
 
         <service id="session.flash_bag" class="Symfony\Component\HttpFoundation\Session\Flash\FlashBag" />
+        <service id="Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface" alias="session.flash_bag" />
 
         <service id="session.attribute_bag" class="Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag" />