Add possibility to manage innerBlocks while migrating deprecated blocks.

[jorgefilipecosta]

This PR was extracted from #5452, to make reviewing easier as this ones just focus on changes to the blocks API.

Description

Before the migrate function received the existing attributes and returned the new attributes now, migrate receives an object with attributes and innerBlocks and returns another object with the migrated attributes & innerBlocks.
This allows, for example, to migrate an attribute to an innerBlock, or the contrary move an inner block to be an attribute.
These changes were used to migrate existing cover image blocks to the nested version.

How Has This Been Tested?

To test the cover image migration merge this changes in a temporary branch with the nesting in cover image PR.

Add a post with following code:

<!-- wp:cover-image {"url":"http://via.placeholder.com/350x150","contentAlign":"right","id":14,"dimRatio":0} -->
<div class="wp-block-cover-image has-right-content" style="background-image:url(http://via.placeholder.com/350x150)">
    <p class="wp-block-cover-image-text">aaaaa</p>
</div>
<!-- /wp:cover-image -->

<!-- wp:cover-image {"url":"http://via.placeholder.com/350x150","id":14} -->
<div class="wp-block-cover-image has-background-dim" style="background-image:url(http://via.placeholder.com/350x150)">
    <p class="wp-block-cover-image-text">aaaaa</p>
</div>
<!-- /wp:cover-image -->

<!-- wp:paragraph {"fontSize":56} -->
<p style="font-size:56px">dsfdsfs</p>
<!-- /wp:paragraph -->

Open the post in the visual editor and verify no invalid block error appears and the cover image blocks and paragraphs were migrated.

[youknowriad]

Funny that to avoid breaking changes, we're introducing another breaking change here :) (especially the return value)

Options here:

• We're ok with this, assuming people didn't use migrate yet.
• We're not ok and we try to support the old API. Here's how I would achieve it:
  • Instead of passing an object, pass the innerBlocks as a second argument.
  • If you want to update both attributes and innerBlocks, return an array with two elements, if you return just an object assume we update only the attributes.

Thoughs @aduth @mcsf

[mcsf]

I'm still forming an opinion, but leaning toward number 2. In practice, I doubt that migrate has been used a lot in the wild up till now, but that's entirely speculative (thus epistemically irresponsible? 😄), and I think it's bad karma to be changing a migration API at this stage.

[jorgefilipecosta]

This PR was updated to be back compatible I followed the suggestion and now we receive two arguments and return an array with attributes and inner blocks or an object with just the attributes.

[youknowriad]

Minor: Can we simplify the logic here or is it too clever?

const [ migratedAttributes, migratedInnerBlocks = innerBlocks ] = castArray( migratedBlockAttributesAndInnerBlocks );

[jorgefilipecosta]

I like this version castArray simplifies the logic, I updated to use it :)

[youknowriad]

LGTM 👍

[jorgefilipecosta]

Thank you for the reviews @youknowriad and @mcsf!