Place "Write mode" functionality behind a Gutenberg experiment

[annezazu]

In light of user testing and current known limitations outlined in #60021, let's change Write mode to an experiment for now to better contain the experience and give time for the feature to evolve before rolling it out more broadly again. In the referenced issue, I put together a list of proposed Tasks to stabilize Write mode that can be discussed further there.

cc @ntsekouras @youknowriad @mcsf for help!

[ntsekouras]

What would this mean in terms of the modes? If the experiment is not enabled, should we only have the design mode or you're thinking of bringing back the select mode we had in place of the write mode?

[youknowriad]

I think the idea is just to hide the tool selector entirely

[annezazu]

I don't have a strong direction here but either hiding it (if that's easier) or returning it to the prior setup of Edit vs Select options.

[richtabor]

Yes, just hiding it entirely and ensuring "Design" mode is set. I don't think it's worth backtracking to Edit / Select.