Adds an identifier prop to the RichText component for captions, allowing annotation

[mhkuu]

What?

This PR adds an identifier prop to the RichText component for audio, embed, gallery, image, table, and video captions.

Why?

This PR is a follow-up to this older PR. With this PR, we allow to annotate textual content in audio, embed, gallery, image, table, and video captions using the core/annotations store. In Yoast SEO, we use this annotation functionality for our highlighting functionality, e.g., in our keyphrase distribution assessment.

How?

This PR opens up captions to annotation by adding an identifier prop to the RichText component for audio, embed, gallery, image, table, and video captions.

Testing Instructions

1. Open a new Post.
2. Insert a paragraph block and an image block.
3. Add a caption to the image.
4. Add the text "This is a test for Gutenberg." to the paragraph block, as well as to the image caption.
5. Add the following code to your console. Replace the blockClientId with the id of the paragraph block (you can find that in the data-block attribute when you inspect the underlying HTML):

wp.data.dispatch( "core/annotations" ).__experimentalAddAnnotation({
  type: 'ANNOTATION_ADD',
  id: 'test',
  blockClientId: 'd10cd677-8236-4e0a-8062-f96e9f17ca2c', // change this!
  richTextIdentifier: 'content',
  source: 'test',
  selector: 'range',
  range: {
    start: 19,
    end: 28
  }
})

6. Confirm the word "Gutenberg" is marked with yellow in the paragraph block.
7. Add the following code to your console. Replace the blockClientId with the id of the image caption block:

wp.data.dispatch( "core/annotations" ).__experimentalAddAnnotation({
  type: 'ANNOTATION_ADD',
  id: 'test',
  blockClientId: 'd10cd677-8236-4e0a-8062-f96e9f17ca2c', // change this!
  richTextIdentifier: 'caption',
  source: 'test',
  selector: 'range',
  range: {
    start: 19,
    end: 28
  }
})

8. Confirm the word "Gutenberg" is marked with yellow in the image caption.
9. Without this PR, the word "Gutenberg" would be highlighted in the paragraph content, but not in the image caption content.
10. Repeat the above steps for audio, embed, gallery, table, and video blocks.

Testing Instructions for Keyboard

NA

Screenshots or screencast

NA

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @mhkuu! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[youknowriad]

I think we have more blocks with captions: table, gallery, embed. Should we be doing the same there for consistency?

[mhkuu]

I think that does make sense, good point 👍 I'll look into those and will adapt the PR accordingly.

[youknowriad]

LGTM 👍