As we won't do T156675: Allow editing <pre> inline, without a popup dialog/inspector for a long while, we should make the experience of editing <pre> nodes (not indent-pre blocks) a bit nicer.
We should allow:
- make the editing experience nicer and more performant;
- explain to users what they're editing, somehow;
- converting from <pre> to indent-pre; and
- allow creating new ones, maybe.
(Split off from T159231: <pre> text displays contents in the surface without the newlines, which is confusing..)