Feature: #84990 - Mark broken file links in RTE

Description

Links to files that were detected as broken by the system extension linkvalidator are now marked accordingly in the RTE via \TYPO3\CMS\Core\Html\Event\BrokenLinkAnalysisEvent .

Those links are now marked with extra markup (yellow background with red border) in RTE.

The procedure for marking the broken links in the RTE is as follow:

1. RTE content is fetched from the database. Before it is displayed in the edit form, RTE transformations are performed.
2. The transformation function parses the text and detects links.
3. For each link, a new PSR-14 event is dispatched.
4. If a listener is attached, it may set the link as broken and will set the link as "checked".
5. If a link is detected as broken, RTE will mark it as broken.

The implementation for checking file links is supplied by the system extension linkvalidator.

Other extensions can use the event to override the default behaviour.

Impact

The behaviour stays the same as before unless the system extension linkvalidator is installed.

If linkvalidator is installed and regularly checks for broken file links, those links will be marked in the RTE.

If linkvalidator is used, it is recommended to use the scheduler to regularly check for broken links.