[BUGFIX] Consider path links with leading slash as `url`

`ext:redirects` allowed to redirect to absolute target
path. With #97159 the target field TCA type has changed
and did not allowed to enter a path identifier since then.

This change lets the `LinkService` resolve a path identifier
not matching a file or folder as type `URL` and restore the
ability to create path based redirect target links.

Additionally, it's now possible again to link directly `/`
again in link fields or RTE fields (#97693).

The underlying solution is that "old-school" links to files
(before FAL) to e.g. "fileadmin/mydownload.exe" is now
actually treated as an external URL, and not as a automatically
detected file anymore.

Resolves: #101083
Resolves: #97693
Related: #97159
Releases: main, 12.4
Change-Id: Ib6a1503137b1c424c55aa3785f6c5d260b5a3aad
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79734


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>