Add return types to Twig implementations

Problem/Motivation

In 11.2 we tightened up some of our deprecation skips: #3523383: Tidy up and tighten deprecation skips

%Method "Twig\\Extension\\ExtensionInterface::[^"]+" might add "array" as a native return type declaration in the future. Do the same in implementation "Drupal\\Core\\Template\\TwigExtension" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\Loader\\FilesystemLoader::findTemplate\(\)" might add "\?string" as a native return type declaration in the future. Do the same in child class "Drupal\\Core\\Template\\Loader\\FilesystemLoader" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\Loader\\LoaderInterface::exists\(\)" might add "bool" as a native return type declaration in the future. Do the same in implementation "Drupal\\Core\\Template\\Loader\\StringLoader" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\Node\\Node::compile\(\)" might add "void" as a native return type declaration in the future. Do the same in child class "Drupal\\Core\\Template\\TwigNodeCheckDeprecations" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\Node\\Node::compile\(\)" might add "void" as a native return type declaration in the future. Do the same in child class "Drupal\\Core\\Template\\TwigNodeTrans" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\NodeVisitor\\NodeVisitorInterface::[^"]+" might add "[^"]+" as a native return type declaration in the future. Do the same in implementation "Drupal\\Core\\Template\\RemoveCheckToStringNodeVisitor" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\NodeVisitor\\NodeVisitorInterface::[^"]+" might add "[^"]+" as a native return type declaration in the future. Do the same in implementation "Drupal\\Core\\Template\\TwigNodeVisitor" now to avoid errors or add an explicit @return annotation to suppress this message%
%Method "Twig\\TokenParser\\TokenParserInterface::[^"]+" might add "[^"]+" as a native return type declaration in the future. Do the same in implementation "Drupal\\Core\\Template\\TwigTransTokenParser" now to avoid errors or add an explicit @return annotation to suppress this message%

In 12.0 we can remove these skips and add the return types to avoid issues if/when Twig 4 is released.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet