fix: update snapcraft requirements, fix ci multipass errors

[VerteDinde]

This PR makes a number of improvements:

• Moves strict confinement to being the default mode
• Updates the template .yaml for strict confinement to include gnome extensions
• Updates the base template to distinguish between strict and classic confinement, and changes the needed stage files per confinement method
• Adds tests for confinement and new interfaces
• Modifies how our snaps are built in CI, so that CI testing actually works with multipass

Outside of this PR, I'd like to add the ability to add custom layouts (we already have to ability to add custom slots), but given the amount of changes in this PR alone, that work can be broken into a separate PR.

Note: This PR enables a user to build a snap locally by setting --provider:host as an argument. This is working in CI with multipass, but unfortunately does not yet work locally. I'm also working on fixing that in a separate PR.

Closes #113, sets the groundwork for closing #83

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

[codecov]

Codecov Report

Merging #144 (073aa83) into master (eb0998b) will increase coverage by 4.38%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #144      +/-   ##
===========================================
+ Coverage   95.61%   100.00%   +4.38%     
===========================================
  Files           8         8              
  Lines         251       259       +8     
===========================================
+ Hits          240       259      +19     
+ Misses         11         0      -11     

Impacted Files	Coverage Δ
src/launcher.js	100.00% <100.00%> (ø)
src/snapcraft.js	100.00% <100.00%> (+23.33%)	⬆️
src/yaml.js	100.00% <100.00%> (ø)
src/index.js	100.00% <0.00%> (+5.35%)	⬆️
src/hooks.js	100.00% <0.00%> (+7.14%)	⬆️

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