Add a `publish` subcommand to the xtask (which performs a dry-run by default)

[jessebraham]

Seems to work as far as I can tell 😅

[jessebraham]

Considered adding a publish command instead which optionally allows for dry run, but I'm afraid I'll forget the argument. Can invert the logic I guess, but something like cargo publish --publish seems weird 😅 Open to suggestions here

[jessebraham]

Okay I ended up changing it to publish, which performs a dry-run by default but can optionally be supplied --no-dry-run. This command is a bit spicy but will be handy I guess 😅