Insert, highlight and align line numbers in xref output

Insert, highlight and align line numbers in xref output

* lisp/progmodes/etags.el (xref-location-line): Specialize for
xref-etags-location.

* lisp/progmodes/xref.el (xref-location-line): New generic method.
(xref-file-location): Add reader for the line slot.
(xref--location-at-point): Skip to the `xref-location' property.
(xref--collect-reference): Drop the line number from description.
(xref--insert-xrefs): Insert, highlight and align line numbers.