Feature: #93209 - FAL: Add getFile() to TYPO3CMSCoreResourceFolder

See forge#93209

Description

The FAL \TYPO3\CMS\Core\Resource\Folder object now contains a new convenience method getFile().

The \TYPO3\CMS\Core\Resource\FolderInterface does not contain the definition yet, as this would be a breaking change, thus, a comment is added to make sure the interface gets this addition in TYPO3 v12 as well.

Impact

When dealing as a developer with FAL Folder objects, the method $folder->getFile("filename.ext") can now be used instead of $folder->getStorage()->getFileInFolder("filename.ext", $folder).