snap: add snap manifest

[soumyaDghosh]

• PR Description

Adds the snap package manifest

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

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There is a package for lazygit in the snap store, but that's old and seems to be unmaintained. It'd be great if lazygit can be updated and upstreamed. Thanks!

[ecruzolivera]

Hello, I'm extremely interested in this, is there any chance that this can be accepted? I have experience with snaps, and I'm willing to help if needed.