Improve SHR/EWW support for 'visual-wrap-prefix-mode'

Improve SHR/EWW support for 'visual-wrap-prefix-mode'

* lisp/visual-wrap.el (visual-wrap--apply-to-line): Use
'add-display-text-property' so we don't clobber other display
properties.
(visual-wrap--content-prefix): Remove special-case for spaces-only
indent prefix; this was an attempt to be helpful for variable-pitch
fonts, but in practice just interferes with matters.  This case now
falls back to the one immediately following it (return the string of
spaces).  Use 'string-pixel-width' instead of 'string-width'.

* lisp/net/shr.el (shr-indent): Set 'shr-prefix-length' here to help
keep track of the prefixes of nestedly-indented elements.  Set the
specified space width in terms of the default width of the current face.
* lisp/net/shr.el (shr-adaptive-fill-function): Use 'shr-prefix-length'
as set above to return a fill prefix.

* lisp/net/eww.el (eww-render): Enable 'visual-wrap-prefix-mode'
alongside of 'visual-line-mode'.
(eww-mode): Set 'adaptive-fill-function' to
'shr-adaptive-fill-function'.

* etc/NEWS: Announce this change (bug#72485).

(cherry picked from commit a876c4d7a17df152e3e78800c76ddf158f632ee5)