Add pattern syntax for optional trailing slash

[tp971]

I proposed this idea in #3258 (comment) to handle cases where trailing slashes are relevant. Basically, allow defining a route with a pattern like "/some/path/?" that matches both "/some/path" and "/some/path/". I also proposed to add a method to check whether the URI has a trailing slash, but that can also be done with request.uri().path().ends_with('/'), so idk whether we actually need this.