Show "readable" status in the EWW mode line

Show "readable" status in the EWW mode line

* lisp/net/eww.el (eww-display-html): Check whether a readable form of
the document exists; if not, don't set ':readable' to t.
(eww-readable): Check whether a readable form of the document exists; if
not, warn the user and don't add to history.
(eww-readable-dom): Return nil if no readable form exists.
(eww-mode): Check the ':readable' property of 'eww-data' to show
"readable" state in the mode line.

* test/lisp/net/eww-tests.el
(eww-test/readable/default-readable/non-readable-page): New test
(bug#78958).

(cherry picked from commit d3c75a3d2de6aa2609b1afef0ae2d1a6e833e9bc)