Social Links: Set the default protocol to 'https' if not specified

[Mamaduka]

What?

Resolves #21699.
Alternative to #30100.

PR tries to use https as the default protocol if the URL doesn't appear to contain schema and it's not a relative link starting with //.

Why?

If URL has no schema esc_url, prepend it with http://. All social media platforms use https://, so it makes sense to be the default fallback.

How?

I've used slightly modified logic from esc_url. Schema is checked using parse_url and code only checks for relative links with // prefix.

Testing Instructions

1. Open a Post or Page
2. Insert the example block (snippet below).
3. Save and view the post.
4. On the front-end inspect block elements and confirm that only WordPress social link has a default protocol.

Snippet

<!-- wp:social-links -->
<ul class="wp-block-social-links"><!-- wp:social-link {"url":"//twitter.com/WordPress","service":"twitter"} /-->

<!-- wp:social-link {"url":"profiles.wordpress.org/wordpress/","service":"wordpress"} /-->

<!-- wp:social-link {"url":"http://github.com/WordPress","service":"github"} /--></ul>
<!-- /wp:social-links -->

Screenshots or screencast

[aristath]

LGTM 👍

[Mamaduka]

Thanks for the review, Ari ❤️