LeptosOptions: provide the CSS path

[jaskij]

As in the title - it'd be great if LeptosOptions provided a function (or member?) with the path to the CSS file generated.

From what I can see, the generated CSS file is put in /pkg/{output_name}.css. It could be a simple helper function, currently I have:

fn css_path(leptos_options: &LeptosOptions) -> String {
    let mut path = String::with_capacity(200);
    path.push('/');
    path.push_str(&*leptos_options.site_pkg_dir);
    path.push('/');
    path.push_str(&*leptos_options.output_name);
    path.push_str(".css");

    path
}