Support using ripgrep in project-find-regexp and friends

Support using ripgrep in project-find-regexp and friends

Performance results vary here.  Some projects and search terms
don't see much of a change, but for some (including Emacs sources
checkout and case-insensitive search) the switch to ripgrep shows
~2-3x speed improvement.  Another piece of anecdata here:
https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00802.html

* lisp/progmodes/xref.el (xref-search-program-alist)
(xref-search-program): New user options.
(xref-matches-in-files): Use them.