[BUGFIX] Correctly consider nested tags in frontend HTML parser

Nested tags starting with the same literals, like `<s><span>...`
or `<a ...><abbr>...` are not correctly nested due to a flaw in
identifying proper start and end of HTML tags.

Resolves: #91194
Releases: master
Change-Id: I2029c8e01d66e5286790fd04a259153cd130c753
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64409


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>