Add snap packaging

[artivis]

This PR adds snap packaging together with a ci workflow that builds the snap on pr/merge and publishes it on merge.

The snap is already available on the Store at the latest release (18.4.0), it is however unlisted at the moment. To give it a shot:

snap install atuin --beta

I've already used it a little while and it seems to work fine.
But I'm not (yet) an atuin power user and I've only been using it with bash.
However zsh/resh/fish should be supported and I'd love to get some feedback about those other shell.

A couple notes:

• in snaps, $HOME is re-mapped and atuin's config files and such are located at $HOME/snap/atuin/current/.
• snaps are sandboxed, therefore to import an existing history the user must connect the appropriate snap interface (e.g. snap connect atuin:dot-bash-history). The import command in the snap has a wrapper that informs the user if no such connection is established.
• the snap ships ble.sh and Bash-preexec for convenience (e.g. source /snap/atuin/current/usr/share/blesh/ble.sh)

I currently own the app on the store and I'd be happy to transfer it to you if you're interested in this pr!

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[ellie]

Hey! Thank you for this, but I'd rather not take this on board.

We intentionally only build + publish binaries for different operating systems and architectures at the moment, and do not package for individual linux distros or package managers.

Specifically for snap/flatpak/similar, I'd rather avoid having to maintain something that aggressively sandboxes and is likely to have unique problems there