Z 1200/replace in buffer (#2922)

This is still WIP, mostly pending styling. I added a pretty rudimentary
text field and no buttons whatsoever other than that. I am targeting a
Preview of 09.13, as I am gonna be on PTO for the next week.

I dislike the current implementation slightly because of `regex`'s crate
syntax and lack of support of backreferences. What strikes me as odd wrt
to syntax is that it will just replace a capture name with empty string
if that capture is missing from the regex. While this is perfectly fine
behaviour for conditionally-matched capture groups (e.g. `(foo)?`), I
think it should still error out if there's no group with a given name
(conditional or not).
Release Notes:

- Added "Replace" functionality to buffer search.