[BUGFIX] Avoid overriding of custom `doktypesToShowInNewPageDragArea`

With the introduction of BeforeLoadedUserTsConfigEvent, it is now
possible to add user tsconfig before global file user.tsconfig is
evaluated. The registry-like `doktypesToShowInNewPageDragArea` list
always overrides possible items added in this event. The fix is to use
the `addToList` modifier, so that default doktypes are appended.

Resolves: #103584
Related: #101818
Releases: main
Change-Id: I4cc90d2a89e4619849cd41085d091903353efe72
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83726


Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>