feed_links and feed_links_extra should have filters

[topdownjimmy]

There ought to be a way to modify the way feed links are displayed by the feed_links function. Currently there is, for instance, no way to change the separator from » to something else.

[williampatton]

Could a comment be added above the files to indicate that they are new?

[SergeyBiryukov]

Yes, new filters need to be documented as per the ​documentation standards.

This code change updates original changes from the trac ticket below, to match the current code from WordPress Trunk. Everything should match what the original patch submitter added, with the exception of a touch of extra punctuation with the PHPDoc block and the version. Once the PR is ready to be accepted, I can update the NEXT value for the accepted release version.

Trac ticket: See https://core.trac.wordpress.org/ticket/43225

[tw2113]

Issued a PR on Github to refresh @topdownjimmy's original work.

[SergeyBiryukov]

In 58821:

Feeds: Introduce feed_links_args and feed_links_extra_args filters.

This allows for more flexibility in modifying how feed links are displayed by the feed_links() and feed_links_extra() functions, including, for example, a way to change the » separator to something else.

Follow-up to [10377], [33838], [33839], [53125], [54161].

Props topdownjimmy, tw2113, williampatton.
Fixes #43225.

Thanks for the PR! Merged in r58821.