Modernize swift scripts in Library/Homebrew/cask/utils

[NSAntoine]

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The following PR modernizes the swift scripts in Library/Homebrew/cask/utils (trash.swift and quarantine.swift), by using features that are Swift-like, such as guard statements.

The functionality and usage of the 2 scripts remains the same

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @Serena-io!