Refactor find-appcast dev script

[danielbayley]

Related to #174192.

• find-appcast

Script simplified, and refactored to more reliably, and efficiently parse the property list to extract the feed URL.

---

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.
• brew style --fix <cask> reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused (add your cask's name to the end of the search field).
• brew audit --cask --new <cask> worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes, including zap stanza paths.

---

[bevanjkay]

Thanks @danielbayley - can we please wait for the rest of the PRs to be reviewed before opening any more. As it's a lot for maintainers to get through.

[danielbayley]

Thanks @danielbayley - can we please wait for the rest of the PRs to be reviewed before opening any more. As it's a lot for maintainers to get through.

Ok will hold off on the others for now. Sorry, I tend not to waste cycles consulting the magic 8 bot, and so remain highly productive…