POC to redirect fragments/anchors

Re-roll from latest 1.x-dev/1.8 and for D9 (update renamed 'user.private_tempstore' service).

Improved to use only temp storage and not need to set a 'fragment_redirects' flag in the query string.

I find no branch where the recent three patches apply.

Another reroll of patch in #7, fixing syntax errors.

This patch now depends on the fragment being stored in the hash, and this only works if "Retain query string through redirect." is off. If it is on the fragment is not stored. Se 🐛 Redirect hashes created when not retaining the query string RTBC

This is a good clean solution to redirecting fragments. It is far better than making a separate module. If the fragment reaches the server we should be able to use it. I see it is a major feature request, and have changed it to "Normal".

On the other hand, module maintainers are often reluctant to include additional features. New features may mean more work for them. If it is not included in a published release, a solution needing a large patch on multiple files causes a maintenance headache. I'd like to see some positive reaction from the maintainers before putting more work into it.