Social Link: Automatically prepend emails with mailto protocol

[Mamaduka]

What?

Resolves #21876.

PR updates Social Links renderer to prepend emails with mailto: protocol if it's not set.

Why?

The current renderer results in an invalid link prefix - http://info@example.com/. This was happening before #42167, because of esc_url.

Testing Instructions

1. Open a Post or Page.
2. Insert an example block snippet.
3. Confirm that email without a prefix has one.
4. There's no double prefix for the second email.

<!-- wp:social-links -->
<ul class="wp-block-social-links"><!-- wp:social-link {"url":"gutenberg@example.com","service":"mail"} /-->

<!-- wp:social-link {"url":"mailto:gutenberg@example.com","service":"mail"} /--></ul>
<!-- /wp:social-links -->

[Mamaduka]

Thanks, Nik!

[mburridge]

Added the Needs Dev Note label in case this needs a dev note (either individual or as part of a "misc" dev note) for WP 6.1 release.

[Mamaduka]

It is a small enough change for Block Editor misc dev notes.

Dev Note

Starting from WordPress 6.1, the Social Links block will automatically prepend emails with the mailto: if the protocol is missing.

For more info, see #42186.