Initial attempt at xref forward and backward commands

[brett-lempereur]

Since switching to eglot, in large codebases I've noticed that Xref lacks a good way to quickly traverse back up through the stack. By the time you're ten or twenty files deep it's quite painful. Consult defines consult-global-mark which nearly works for navigating the xref stack (try (consult-global-mark (car (xref--get-history)))), but doesn't properly update the state of the stack.

This is an initial attempt at defining consult commands for navigating backward and forwards through the stack, and I'm opening it early to gauge interest and for discussion. It's probably not very idiomatic and shipping it might need modifications to xref itself, so we don't need to hit a private API for the history.

[brett-lempereur]

Moved this into an extension package and fixed it up. Something like it probably belongs in Emacs and Consult, but this will do for now: https://github.com/brett-lempereur/consult-xref-stack.