Support regexp search with the etags backend.

* xref.el (xref-read-identifier-from-minibuffer [etags]): If the
text starts with " treat it as regexp.  A "regexp identifier" is
reperesent as a list of the form (rx REGEXP).
(xref-find-definitions [etags]): Handle regexps.
Bind . and , to reduce finger movement.

Suggested by Johan Claesson.

* xref.el (xref--xref-buffer-mode-map): Bind ./, to
xref-next-line/xref-prev-line.