minor #4420 Add $this return type to Template::unwrap (ruudk)

This PR was merged into the 3.x branch.

Discussion
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Add `$this` return type to Template::unwrap

See https://phpstan.org/writing-php-code/phpdoc-types#static-and-%24this

This way, PHPStan understands that the returned template is the exact same instance, and not just a Template.

Commits
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92e4989002 Add `$this` return type to Template::unwrap

src/Template.php

@@ -318,6 +318,7 @@ abstract class Template
 
     /**
      * @internal
+     * @return $this
      */
     public function unwrap(): self
     {