Allows desktop apps to open mailto links

[jjvn84]

Added an option to the WebViewBuilder navigation handler to open links starting with "mailto:".

webview = webview
            .with_web_context(&mut web_context)
            .with_bounds(wry::Rect {
                position: wry::dpi::Position::Logical(wry::dpi::LogicalPosition::new(0.0, 0.0)),
                size: wry::dpi::Size::Physical(wry::dpi::PhysicalSize::new(
                    window.inner_size().width,
                    window.inner_size().height,
                )),
             })
            .with_transparent(cfg.window.window.transparent)
            .with_url("dioxus://index.html/")
            .with_ipc_handler(ipc_handler)
            .with_navigation_handler(|var| {
                // We don't want to allow any navigation
                // We only want to serve the index file and assets
                if var.starts_with("dioxus://") || var.starts_with("http://dioxus.") {
                    true
                 } else {
                    if var.starts_with("http://") || var.starts_with("https://") {
                        _ = webbrowser::open(&var);
                     } else if var.starts_with("mailto:") {
                        _ = open::that(&var);
                     }
                    false
                }
 })

This adds a dependency to the open crate.
We can also join the two if branches and use the open crate to open http, https, and mailto links. Is there a reason to keep them separated?

Fixes #2991

[jkelleyrtp]

It seems like open is the better of the two crates, so I switched your PR to just use open, removing the webbrowser dependency. Thanks for the PR!