feat: add filter to social icons map

[leogermani]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

COPIED FROM #2170. Props to @philipjohn Just did a minor change.

Introduces a new hook for filtering the social icons map used to determine which logo to show in the social links menu.

With the introduction of Mastodon and the plethora of instances that could be used, including self-hosted instances, it would good to allow plugins and/or child themes to filter the social icons map to add additional mastodon instance addresses. This filter achieves that and can be used like so:

/**
 * Add our mastodon server to social icon map in Newspack.
 * 
 * @param array $social_icons_map Array of matches for social icons.
 * 
 * @return array Modified array of matches for social icons.
 */
function lichfieldlive_newspack_social_icons_map( $social_icons_map ) {
	$social_icons_map['mastodon'] = array_merge(
		$social_icons_map['mastodon'],
		[
			'social.lichfieldlive.co.uk',
		]
	);
	return $social_icons_map;
}
add_filter( 'newspack_social_icons_map', 'lichfieldlive_newspack_social_icons_map' );

It could also be used, for example, by Mastodon plugins such as the ActivityPub plugin to ensure the connected server is recognised. A further change to the Newspack Theme could add a compatibility layer for plugins like ActivityPub.

How to test the changes in this Pull Request:

1. Create or edit the Social Links Menu
2. Add a custom link to a Mastodon server account that does not already exist in the Newspack Theme's list (e.g. https://social.lichfieldlive.co.uk/@news)
3. Using a small plugin, or child theme, create a function to add the social.lichfieldlive.co.uk domain to the social icons map (see the above example)
4. Observe that the new item in the Social Links Menu has the Mastodon icon correctly applied (see screenshot below)

Note that as the social icons map is a static class variable it wasn't possible to add the filter to that array directly to only focus on the Mastodon map, so instead I had to filter the whole list.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 1.83.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.83.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀