Register Valent as a system handler for tel: links

[CaragC]

Currently, clicking a tel: link in the browser does not open Valent on the desktop

Feature Request:

• Register Valent as an x-scheme-handler/tel in its desktop file, so users can choose it as the handler for tel: links.
• Ensure that when the tel: protocol is handled, Valent launches and processes the URI as expected (e.g., opening the dialer interface or forwarding the request).

[andyholmes]

This was removed (awhile back) in 53c830d:

fix: remove scheme handlers from .desktop file

Remove the `x-scheme-handler/sms` and `x-scheme-handler/tel` from the
.desktop file. These are not yet used anyways, but may cause problems
on Phosh-based desktops that don't have defaults set for those types.

I'm not sure if the situation has changed since then (Dec 2022), but I wouldn't want to break Phosh/GNOME Mobile 😬