Block API: Set alias_name on block class instances.

[tjcafferkey]

Trac ticket:

Consider the following markup:

<!-- wp:social-links -->
<ul class="wp-block-social-links">
	<!-- wp:social-link {"url":"Tumblr.com","service":"tumblr"} /-->
	<!-- wp:core/social-link/wordpress {"url":"wordpress.com","service":"wordpress"} /-->
</ul>
<!-- /wp:social-links -->

The second social link has an alias core/social-link/wordpress which we need to render correctly on the frontend with the correct CSS class names.

All WP_Block_Type's are registered under their canonical form, meaning aliases are not in the registry. So when we are rendering an alias block type we need to know what the alias is so we can generate the appropriate class names for it, for the above mark up this would be: wp-social-link wp-social-link-wordpress wp-block-social-link.

This PR attempts to solve this by tracking the alias when we look it up in the block registry. When we return the canonical WP_Block_Type it will have an $alias_name property so we know which alias it is representing.

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