[BUGFIX] Treat link handler links as internal URLs

The RteHtmlParser will now check if a scheme found by parse_url()
matches a configured typolinkLinkHandler. If this is the case the
link will be treated as internal.

Resolves: #49036
Releases: 6.3, 6.2, 6.1
Change-Id: Ie4e6bded7a1c4d0c61e4f1a20bc23115612c1519
Reviewed-on: https://review.typo3.org/30975
Reviewed-by: Markus Klein
Tested-by: Markus Klein