Write a proposal for IPv6 "Happy Eyeballs" on Tor clients

Teor has asked for updates in https://github.com/torproject/torspec/pull/61, but this PR has already been merged.

I have created a new PR for his suggestions: https://github.com/torproject/torspec/pull/63

I don't believe you can add to a merged PR so that's why a new PR exists.

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