New user option: project-vc-extra-root-markers

New user option: project-vc-extra-root-markers

* lisp/progmodes/project.el: Commentary update.
(project-vc, project-vc-include-untracked, project-vc-name):
Update docstrings.  Rename 'VC project' to 'VC-aware project'.
(project-vc-extra-root-markers): New option (bug#41572).
(project-try-vc): Use it.  Construct a single regexp from all and
validate it using the MATCH argument of 'directory-files'.  Call
'locate-dominating-file' directly.
(project-ignores): Support VC-aware project instances with nil
value of VC backend.

* test/lisp/progmodes/project-tests.el (project-vc-recognizes-git)
(project-vc-extra-root-markers-supports-wildcards)
New tests.
(project-tests--this-file): New variable.