[FEATURE] Introduce Feature Toggles

A new API class "Features" allows to check if a feature is enabled/disabled
for a certain installation.

This API method can be used to have admins switch to new features explicitly
or disable legacy functionality.

Examples for using the feature toggles in the future:
- Do not load TCA for pages_language_overlay after DB migration
- Do not parse HTML for legacy <link> tag anymore
- Explicitly enable new Logging API in DataHandler (skipping sys_log calls)

Resolves: #83429
Releases: master
Change-Id: I5da8f66e593e311c83fefe5fe9edb503a885943b
Reviewed-on: https://review.typo3.org/54529


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>