Add xwidget-webkit-isearch-mode

Add xwidget-webkit-isearch-mode

* etc/NEWS:
* doc/emacs/misc.texi: Document changes

* lisp/xwidget.el (xwidget-webkit-mode-map): Bind C-s and C-r to
webkit isearch.
(xwidget-webkit-isearch--string, xwidget-webkit-isearch--is-reverse):
New buffer-local variables.
(xwidget-webkit-isearch--update, xwidget-webkit-isearch-erasing-char)
(xwidget-webkit-isearch-printing-char, xwidget-webkit-isearch-forward)
(xwidget-webkit-isearch-backward, xwidget-webkit-isearch-exit):
New functions.
(xwidget-webkit-isearch-mode-map): New keymap.
(xwidget-webkit-isearch-mode): New minor mode.