Add Feature Flag for the immersive Wiki editor

What does this MR do and why?

This MR introduces the :wiki_immersive_editor Feature flag. To reduce MR size, at this point it does nothing but add the immersive CSS class to the rich text editor. All functional features will be added in a later MR

References

Screenshots or screen recordings

no UI changes

How to set up and validate locally

1. Run rails c, then run Feature.enable(:wiki_immersive_editor)
2. Open or create a project with a wiki page
3. open an edit interface by adding a new page or clicking "edit" on an existing one.
4. If you're seeing the plain text editor, click "switch to rich text editing"
5. Confirm that the rich text editor's container has the immersive CSS class

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.