Add AppImage packaging for Linux

[antarktikali]

This adds a new workflow that builds an AppImage. It creates 2 artifacts: libweaver.so and an AppImage for the GUI app. The artifacts are available for each run, but are not uploaded to the releases.

This is also a separate workflow from the windows one (ci.yaml) because that one had some problems currently. They can be combined in the future, but this can be a starting point now.

This is also my first time playing around with the github workflows so any feedback is appreciated! Mostly followed the workflow of Serial-Studio