feat: add cask shell completion

[SMillerDev]

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Still need to check this on Linux, but this adds shell completion artifacts to casks and allows for a way to get the os_version without calling MacOS.full_version. That should make all casks readable on Linux and resolve the comments in https://github.com/orgs/goreleaser/discussions/5563

[MikeMcQuaid]

Seems reasonable! Needs some more tests before merging.

[SMillerDev]

Added tests for the shell completion stanzas, the os_version proved to be difficult to test and frankly not that essential either.