Feature: #84990 - Add event for checking external links in RTE

Description

A new PSR-14-based event \TYPO3\CMS\Core\Html\Event\BrokenLinkAnalysisEvent can be used to get information about broken links set in the rich text editor (RTE).

Up until now, TYPO3 only displayed page links with extra markup (yellow background with red border) if a link would point to an internal page which does not exist anymore.

This was previously checked internally in RTE, and is now moved to an Event Listener to make this check more flexible and interchangeable.

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.

An implementation for external and page links is now supplied by the system extension linkvalidator. External links are currently checked using the existing tx_linkvalidator_links table.

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

Impact

The behaviour for page links stays the same - they are also marked if they do not exist anymore - however, marking the links is now unified and only available when the system extension linkvalidator is installed.

If linkvalidator is installed and regularly crawls for broken links, broken external links will be marked as well.

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