Add Text Columns → Columns transform

[ZebulanStanphill]

Description

This PR adds a one-way Text Columns → Columns transform to make it easier to migrate from the deprecated block. The alignment, custom CSS classes, and content are all preserved.

How has this been tested?

You can test this branch by copy-pasting this markup into a post and trying out the transforms on the Text Columns blocks:

<!-- wp:text-columns {"className":"yoyo"} -->
<div class="wp-block-text-columns alignundefined columns-2 yoyo"><div class="wp-block-column"><p>Content<br/>2nd line in same paragraph<br/><br/>4th line<br/></p></div><div class="wp-block-column"><p>More content</p></div></div>
<!-- /wp:text-columns -->

<!-- wp:text-columns {"width":"center","className":"donut"} -->
<div class="wp-block-text-columns aligncenter columns-2 donut"><div class="wp-block-column"><p>Content<br/>2nd line in same paragraph<br/><br/>4th line<br/></p></div><div class="wp-block-column"><p>More content</p></div></div>
<!-- /wp:text-columns -->

<!-- wp:text-columns {"width":"wide","className":"neato"} -->
<div class="wp-block-text-columns alignwide columns-2 neato"><div class="wp-block-column"><p>Content<br/>2nd line in same paragraph<br/><br/>4th line<br/></p></div><div class="wp-block-column"><p>More content</p></div></div>
<!-- /wp:text-columns -->

Additional notes

Ideally, you would want to automatically transform deprecated blocks into the recommended replacements, but the deprecated blocks API does not currently support migrating from/to a completely different block, so using the transforms API to allow for easy user-initiated transforms is currently the best option.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

Related issues and PRs

• Deprecate Subheading block #9355
• Text Columns block: change title/description to include deprecation notice. #9358

[chrisvanpatten]

Ideally, you would want to automatically transform deprecated blocks into the recommended replacements, but the deprecated blocks API does not currently support migrating from/to a completely different block, so using the transforms API to allow for easy user-initiated transforms is currently the best option.

I don't want to derail this PR, but I'm super curious if this has been discussed in the past. This feels like a great opportunity to improve the deprecation experience.

[ZebulanStanphill]

@chrisvanpatten If it has been discussed before, I don't know about it. 🤷‍♂️ I agree that it would be great to have that kind of functionality, and I'm also curious if it has been discussed before.

[youknowriad]

Can we move the declaration of this transform the the text-columns block instead (to transform) that way when/if we remove the block, we don't forget the transform.

[ZebulanStanphill]

@youknowriad Addressed feedback; the transform is now defined in text-columns/index.js.

[youknowriad]

LGTM 👍 Thanks

[ZebulanStanphill]

@youknowriad This should be added to the 3.7 milestone.