New GNU ELPA package: xref-union - Combine multiple Xref backends

[Sacha Chua]

    Summary: Combine multiple Xref backends
   Requires: emacs-25.1
    Website: https://git.sr.ht/~pkal/xref-union/
 Maintainer: Philip Kaludercic <~pkal/public-inbox@lists.sr.ht>
     Author: Philip Kaludercic <philipk@posteo.net>

This package provides a way to combine multiple Xref source
(e.g. Etags and Eglot) and have the results all at once.

To enable, toggle the `xref-union-mode' minor mode.  If you want to
exclude certain modes, take a look at the user option
`xref-union-excluded-backends'.

You can also manually make use of `xref-union' by adding an object
of the form (union XREF-BACKEND-1 XREF-BACKEND-2 ...) to
`xref-backend-functions'