* src/emacs/lisp/bookmark.el
  (bookmark-menu-confirm-deletion): New defcustom.
  (bookmark-bmenu-execute-deletions): Conditionally confirm.
  (bookmark-delete-all): Add comment explaining why we don't
    use the new confirmation formula here.

(Note: the bulk of the code diff is just reindentation of an otherwise
unchanged `let' expression in `bookmark-bmenu-execute-deletions'.)

Thanks to Oliver Taylor for suggesting the new behavior:

  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>