Improve image carousel accessibility

[dsas]

Fixes #39908

Proposed changes:

Accessibility has been improved by reducing which images receive a tab index and an aria role of button.

This has been done by moving it out of PHP - where it applied the two attributes to all images in the post, and into javascript where it can be added when applying the interactivity which makes the attributes necessary.

An aria-label has been added for good measure.

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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

GB

1. Create a post containing:

• An image block linking to the images attachment page
• An image block not linking to anywhere
• An image block linking to itself
• A tiled gallery
• A slideshow

2. View the post, notice that all of the mentioned images have a role=button and a tabIndex=0 regardless of whether they do act as a button
3. Apply this patch and build
4. View the post again, notice that only the interactive images - those launching a full-window "lightbox" have a role=button and a tabIndex=0 and additionally, they don't have a warning about not being labelled.

TinyMCE

1. Install the classic editor plugin
2. Use the "Add media" button to add images as before, and also a gallery
3. View the post and as before notice that only the interactive images have aria attributes etc.

[github-actions]

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[coder-karen]

Thank you for looking at this and for an excellent solution!

The main change I noticed was in images that did not link to anything - with this PR the role of button and tabIndex have been removed, which is now what we want.

I did notice something else - two different issues - but they were also an issue before:

1. A Tiled Gallery set to link to 'None' still defines a role of button and tabIndex of 0. Both clicking on it and tabbing to it then pressing enter result in no action being taken, which is what I'd assume would be expected. So it looks like they would also need the role and tabIndex removed.
2. An image set to link to an attachment page opens a lightbox if Carousel is enabled if using keyboard and pressing enter, but opens the attachment page if clicking on it (I'd assume it should open the lightbox). If Carousel is not enabled, both clicking on it and tabbing to it then clicking / entering open the attachment page.

These are both quite specific issues, not related to the report anyway, so I'd be happy to create follow-up issues for these.

[dsas]

@coder-karen would rather get this issue landed rather than growing the scope. If you share the links though I will see if I can find some time for those issues too.