Allow toggling "readable" mode in EWW

Allow toggling "readable" mode in EWW

Additionally, add an option to prevent adding a new history entry for
each call of 'eww-readable' (bug#68254).

* lisp/net/eww.el (eww-retrieve):

* lisp/net/eww.el (eww-readable-adds-to-history): New option.
(eww-retrieve): Make sure we call CALLBACK in all configurations.
(eww-render): Simplify how to pass encoding.
(eww--parse-html-region, eww-display-document): New functions, extracted
from...
(eww-display-html): ... here.
(eww-document-base): New function.
(eww-readable): Toggle "readable" mode interactively, like with a minor
mode.  Consult 'eww-readable-adds-to-history'.
(eww-reload): Use 'eshell-display-document'.

* test/lisp/net/eww-tests.el (eww-test--with-mock-retrieve): Fix indent.
(eww-test/display/html, eww-test/readable/toggle-display): New tests.

* doc/misc/eww.texi (Basics): Describe the new behavior.

* etc/NEWS: Announce this change.