Build and publish an AppImage package; use instead of snap as a Linux distro-agnostic option

[JWCook]

Closes #476 and #340

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.93%. Comparing base (f30af6a) to head (021ae64).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #477   +/-   ##
=======================================
  Coverage   74.93%   74.93%           
=======================================
  Files          45       45           
  Lines        4038     4038           
  Branches      308      308           
=======================================
  Hits         3026     3026           
  Misses        959      959           
  Partials       53       53           

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