Support bookmarking 'project-find-regexp' results buffer

Support bookmarking 'project-find-regexp' results buffer

* lisp/progmodes/project.el (xref-backend-apropos)
(xref-backend-context, xref-backend-restore): New methods.
(project-find-regexp, project-or-external-find-regexp): Use
'xref-make-fetcher' instead of a bespoke fetcher function to
facilitate bookmarking the results buffer.

* lisp/progmodes/xref.el (xref-bookmark-make-record): Use
strings for 'format-spec' specifications.
(xref-bookmark-jump): Autoload it.
(xref-show-xrefs): Make DISPLAY-ACTION argument optional.
(xref-make-fetcher): Autoload, and make BUFFER and POINT
arguments optional, default to the current buffer and point.