qubes-run-terminal: Add support for Alacritty

[Undef-a]

Alacritty has support for both starting a new session and just opening a new window. The latter case uses less resources but can only be run if an existing instance of Alacritty is running.

Add Alacritty as a terminal option in qubes-run-terminal then add a wrapper script similar to that used by gnome-console to handle the window creation or first instance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.30%. Comparing base (f0089ce) to head (535027d).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #627   +/-   ##
=======================================
  Coverage   70.30%   70.30%           
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  Files           3        3           
  Lines         495      495           
=======================================
  Hits          348      348           
  Misses        147      147           

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[marmarek]

alacritty msg create-window does not wait for the window to be closed, which is not compatible with starting a disposable this way (it would close immediately).

[Undef-a]

In that situation alacritty wouldn't be running and thus `alacritty msg create-window` will fail and fallthrough to `exec alacritty`. I just tested locally and this works as expected. The new DispVM starts, runs `qubes-run-alacritty` and stays running. Other instances of alacritty can be started through `qubes-run-alacritty`, they will run as new windows. When the last of them is closed the VM shuts down.