Migrate playdate-mirror to cask

[caizixian]

Linked cask PR Homebrew/homebrew-cask#155360

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In the following questions <cask> is the token of the cask you're submitting.

After making any changes to a cask, existing or new, verify:

• [ ] The submission is for a stable version or documented exception.
• [ ] brew audit --cask --online <cask> is error-free.
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Additionally, if adding a new cask:

• [ ] Named the cask according to the token reference.
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• [ ] Checked the cask is submitted to the correct repo.
• [ ] brew audit --new-cask <cask> worked successfully.
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