Add default `T=()` to `rune::Result`

[ModProg]

Allows to do:

type Result<T = (), E = anyhow::Error> = core::result::Result<T, E>;

fn some_test -> Result {
    Ok(())
}

Though I also get not doing this, you really don't save much, but not having to say Result<()>.

[udoprog]

Might be a bit too unusual for my taste.

Strongest precedent I can think of is std::fmt::Result, but then it's set to exactly value. Even there it's a bit odd. 🤔