Add File::create_new convenience c'tor

[jplatte]

A little while ago, std introduced File::create_new as a shortcut for File::options().write(true).create_new(true).open(path).

Describe the solution you'd like
Tokio should follow suit and add the same c'tor for its File type.

Describe alternatives you've considered

Additional context
Happy to provide a PR if there are no concerns about this feature request.

[Darksonn]

Since std has stabilized this method and can no longer change it, there are no concerns with us adding it too.