Trim footnote anchors from excerpts

[ramonopoly]

When the post content has one or more footnotes, the excerpt block picks up the link text, the visually hidden star, as part of the content.

See Gutenberg issue for a description of the problem. ​https://github.com/WordPress/gutenberg/issues/52035

This has been rectified in Gutenberg since: ​https://github.com/WordPress/gutenberg/pull/52518 and needs to be ported to Core.

Backports changes from ​https://github.com/WordPress/gutenberg/pull/52518

Adding a function to wp_trim_excerpt that removes footnotes from content.
Adds tests

Trac ticket: https://core.trac.wordpress.org/ticket/58805

cc @mcsf

Not sure if this is what you had in mind. Feel free to trash this one if so 😄

Also not sure if where I've put things is correct, e.g., different file? Different place in the excerpt formatting chain 🤷🏻

I probably need more tests too. Testing this PR surface a couple of bugs, e.g., ​https://github.com/WordPress/gutenberg/issues/52628

[mukesh27]

Thanks @ramonopoly for ticket and PR. Left some feedback on PR.

[ramonopoly]

Thanks @ramonopoly for ticket and PR. Left some feedback on PR.

Thanks! Appreciate it.

[mcsf]

@ramonopoly: many thanks for the ticket and patch! It looks good and works well in my testing.
@mukesh27: once you're happy with the changes, feel free to approve this one.

Not sure if this is what you had in mind. Feel free to trash this one if so 😄

I've responded in Trac already, but yeah, this is great! :)

[azaozz]

The regexp in the PR seems a little bit too specific, but should work. All feedback seems to have been addressed, +1 to commit from me too.

[isabel_brison]

In 56244:

Editor: trim footnote anchors from post excerpts.

Adds the excerpt_remove_footnotes function to trim footnote anchors from post excerpts.

Props: ramonopoly, costdev, mukesh27, mcsf, azaozz.
Fixes #58805.

Committed in r56244 / ​2beefda.