As an improvement based on the issue Automattic#10662 "Social Icons Widget: allow for alternate feed URLs" this adds some more common feed url components to the get_supported_icons() function. The existing version only matches the existence of "/feed/" in the url being added, as such many other feed URL formats would end up showing with the default link icon instead. Whilst it may still not catch all possilbe feed url formats it will catch many commonly used formats as well as non-permalink versions of WordPress feeds.

Fixed incorrect use of spaces instead of tabs.
Added some additional comments regarding which sources RSS feeds may come from.

There are many possible RSS Feed url formats that can be used to match for the RSS Feed icon, so to reduce repetition of multiple 'feed' entries in the `$social_links_icons` array a separate `$feed_url_formats` now contains the common 'feed' icon and the various feed url parameters. As such it means the 'url' of an item in the `$social_links_icons` array can itself be an array so the code for rendering the widget has been updated to accommodate this.

The previous commit added the various `$social_links_icons` 'feed' items as an array. This cleaned up the code but also added a bit of repetition due to having to check whether a `$social_links_icons` url element was an array and subsequently handle them in two different ways but duplicating some code in the process.

This update changes all url elements in the `$social_links_icons` array to be an array even if there is only one item (as most of them do). This means that the code used to check if a url matches only needs to presume the url element comes as an array. This also enables the possibilty that some of the existing items e.g. Amazon can be reduced to one entry in the `$social_links_icons` array.

…cons` array

This commit reduces the separate entries that exist for Amazon, Google, Discord and WordPress domains into single items with multiple url array items instead.

Now that all supported services use the array format, we can move this whole array back in without creating any confusion.

- Since we know for sure $social_icon['url'] is going to be an array, let's remove the extra check
- Use printf instead of 2 echo
- Add phpcs:ignore to avoid error on output

See:
- https://github.com/Automattic/jetpack/pull/13183/files#r314844476
- https://github.com/Automattic/jetpack/pull/13183/files#r314844856
- https://github.com/Automattic/jetpack/pull/13183/files#r314844732