Social Links Block: Allow plugins to add additional logos/social links

[jasmussen]

In #16897 a "social links" block is likely to land in its first version.

Based on feedback for that first version, we'd like to explore some additional features.

At launch, the Social Links block will have a wide range of icons for most present social networks and sites to link to.

But there will always be some network, service, or tool that the block does not link to, but which the block does not contain. It would be good to explore how to add a simple way for plugins or themes to register new child blocks, so that the core list of blocks can remain relatively slim for maintenance purposes, but still allow flexibility for themers and developers.

[mcsf]

If/once #19887, this becomes automatically possible thanks to the Variations API:

wp.blocks.__experimentalRegisterBlockVariation('core/social-link', {
    name: 'latin-wikipedia',
    title: 'Vicipædia',
    icon: 'smiley',
    attributes: {
        site: 'latin-wikipedia',
        url: 'https://la.wikipedia.org/',
    }
});

[mcsf]

Once #20068 lands, the API becomes wp.blocks.registerBlockVariation.