(Note: I plan to merge this only after we cut the Emacs 30 release 
branch, since it seems a bit too substantial a change to sneak in right 
near the end. However, I think the patch is mostly done aside from one 
remaining issue, so any feedback is very welcome.)

'visual-wrap-prefix-mode' has one small issue: since the wrap prefix is 
just a string, the wrapped text may not line up for variable-width 
fonts. This is mainly in cases like so:

   * here is some text that
     got visually wrapped

If the "* " is variable-width, the second line will probably be indented 
wrong by a few pixels.

The attached patch adds a display spec in this case so that the text 
lines up perfectly. There's currently one problem though: I'm not sure 
how to regenerate the wrap prefix automatically if the face changes. 
It's not hard to handle for 'text-scale-adjust', but I don't know how to 
handle 'global-text-scale-adjust' (or other things that could change the 
face[1]).

Does anyone have any ideas for this part?

[1] There's 'after-setting-font-hook', but that doesn't cover everything 
either.