[url_launcher] Decode URL before passing it to ShellExecuteW.

[ftsui]

• ShellExecuteW expects a path, not a URL. If the path contains non-ascii characters, it needs to be decoded before passing to ShellExecuteW.
• ShellExecuteW seems to be happy with the file:// prefix in the URL

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Pre-launch Checklist

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

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[stuartmorgan]

Thanks for the contribution! Since the checklist isn't complete, I'm assuming this is a work in progress, and marking as a Draft accordingly. Once you've completed all of the steps, please mark it as ready for review.

It also appears that this PR assumes that url_launcher is only used for file: URLs. It's intended for use with any URL, and is very frequently used with schemes like https and mailto; a change that breaks those use cases would not be accepted.