minor symfony#58117 [VarDumper] fix compatibility with Twig 3.12 (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[VarDumper] fix compatibility with Twig 3.12

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

Commits
-------

0f2fee0 fix compatibility with Twig 3.12

Author: xabbuh

src/Symfony/Component/VarDumper/Tests/Fixtures/Twig.php

@@ -34,17 +34,17 @@ protected function doDisplay(array $context, array $blocks = [])
         throw new \Exception('Foobar');
     }
 
-    public function getTemplateName()
+    public function getTemplateName(): string
     {
         return 'foo.twig';
     }
 
-    public function getDebugInfo()
+    public function getDebugInfo(): array
     {
         return [33 => 1, 34 => 2];
     }
 
-    public function getSourceContext()
+    public function getSourceContext(): Twig\Source
     {
         return new Twig\Source("   foo bar\n     twig source\n\n", 'foo.twig', $this->path ?: __FILE__);
     }