New option to confirm deletion in bookmark menu

New option to confirm deletion in bookmark menu

* lisp/bookmark.el (bookmark-menu-confirm-deletion): New defcustom.
(bookmark-delete-all): Add comment explaining why we don't use the new
confirmation formula here.
(bookmark-bmenu-execute-deletions): Conditionally confirm deletion.
Note that the bulk of the code diff here is just reindentation of an
otherwise unchanged `let' expression.

* etc/NEWS: Announce the new option.

Thanks to Lars Ingebrigtsen and Eli Zaretskii for review, and thanks
to Oliver Taylor for suggesting the option in the first place:

  https://lists.gnu.org/archive/html/emacs-humanities/2021-02/msg00022.html
  From: Oliver Taylor
  Subject: Re: [emacs-humanities] Extending Emacs Bookmarks to Work with EWW
  To: Karl Fogel
  Cc: Stefan Kangas, Emacs-humanities mailing list
  Date: Wed, 3 Feb 2021 20:21:59 -0800
  Message-Id: <936D47EA-4D11-452B-8303-971B6386877B@me.com>