Consider converting toggle preview button into a checkbox

https://git.drupalcode.org/issue/same_page_preview-3344837 provides this checkbox. I'm working on this in 📌 On by default, make off-by-default an opt-in setting Closed: duplicate because I forgot this issue exists. I might just mark this one as a duplicate.

Marking as closed. The checkbox was implemented and then already removed in favor of keeping the default preview button. Lol.