feat: add appimage support for linux

[SMillerDev]

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This adds support for the first linux-only cask stanza: app_image.
We should probably add CI for Linux casks, but haven't been able to get that working yet.

[MikeMcQuaid]

We should probably add CI for Linux casks, but haven't been able to get that working yet.

Need CI before this PR is merged.

Where are the app images installed and is that an expected/sensible location?

Maybe we should reuse app for this on Linux (or is that too confusing)?

[SMillerDev]

Maybe we should reuse app for this on Linux

I considered it, but deemed it too confusing. Especially with it not working on macOS.

Where are the app images installed and is that an expected/sensible location?

In ~/Applications, their docs recommend it.
https://docs.appimage.org/user-guide/faq.html#question-where-do-i-store-my-appimages

[MikeMcQuaid]

@SMillerDev sounds good 👍🏻

[SMillerDev]

I'm also considering adding a desktop file for these so the various Linux systems can pick it up easily.

Ref: https://specifications.freedesktop.org/desktop-entry-spec/latest/