Support the 'mailto' scheme

[frewsxcv]

If I click on a mailto link, it should take me to my email client. I don't know if there's a spec for this, but most browsers handle this.

Example: click on https://rwell.org/ and click on 'email'

[frewsxcv]

I'm going to guess the main work here is finding out the default mail client for all platforms. At least on mac, I think that's via:

• https://developer.apple.com/documentation/coreservices/1448824-lscopydefaultapplicationurlforur?language=objc

Might be nice if there was a small crate that handled this across all platforms. e.g. something like:

/// Given an application scheme, determine the path of the app for which it is registered to
fn scheme_app_lookup(scheme: &str) -> Option<Path> { .. }

[badboy]

There's open.

[paulrouget]

Can't that be handled directly by the embedder via WindowMethod::allow_navigation?

Basically, in ports/glutin/window.rs, in fn allow_navigation, we should test if the scheme is mailto:, and if so, use open (already used in components/script) to let the OS handle the URL.

[deg4uss3r]

I made some progress on this by editing components/net/methods.rs mailto: links appear to be working and opening in the default email client (on my Mac) using open::that

[KiChjang]

@deg4uss3r Please hold off your attempt; I agree with @paulrouget, this issue does not sound like it is servo's responsibility but rather the embedder's.