Add snap support

[salix-caprea]

Although snaps are considered Ubuntu-centric, it's nice to have them available on other distros, for example I've been using this snap on Fedora for the last several months without issue.

Built on amd64 Ubuntu 23.04, tested on amd64 Fedora Workstation 38/39.

I didn't bother testing or implementing cross-compiling for i386, if that is desired then it's a lot more work IMO. Another option would be building on native i386.

I did notice that "cyanrip" is already registered on snapcraft.io, if that isn't owned or acquired by this project, then another name should be chosen e.g. cyanrip-snap.

Note: The snap can still be built and installed outside of the store using --devmode, if you elect not to use snapcraft.io. I mention that in the updated README.

Remaining steps:

• Set up project on snapcraft store
• Verify builds correctly on snapcraft.io
• Test build
• Adjust grade to stable in snapcraft.yaml