Overwrite exist method of Twig FilesystemLoader

- Implement exist method for the Symfony\Bundle\TwigBundle\Loader\FilesystemLoader to allow bundle syntax for templates

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | fabpot/Twig#1156, #8665
| License       | MIT

This is option 2 and preferred.