[TASK] Modernize `TypoScriptReferenceController`

The class `TypoScriptReferenceController` used by our code editor get
some minor updates:

* Removed unneded `$xmlDoc` property
* Added missing type declarations
* Resolved open @todo

Resolves: #104352
Releases: main
Change-Id: Iee855f7cf1f2f3fac80ba4f9482473d08420257c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85216


Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>