Add widget-areas endpoint

[jorgefilipecosta]

Description

This PR uses the work done in #14251 with some adaptations to use CPT being implemented in #15014.

This is a step in a try to implement https://github.com/WordPress/gutenberg/blob/add/blocks-in-widget-areas-rfc/docs/rfcs/blocks-in-widget-areas.md

The CPT itself should be review in #15014.

How has this been tested?

I verified executing this code in the browser console:

wp.apiFetch({
    path: '/__experimental/widget-areas',
    method: 'GET',
}).then(console.log);

Returns the widget areas with widgets being represented as legacy widget blocks.

During the first change, a post is created and the sidebar references that post:

wp.apiFetch({
    path: '/__experimental/widget-areas/sidebar-1',
    data: { content: '<!-- wp:columns --><div class="wp-block-columns has-2-columns"><!-- wp:column --><div class="wp-block-column"><!-- wp:paragraph --><p>a</p><!-- /wp:paragraph --></div><!-- /wp:column --><!-- wp:column --><div class="wp-block-column"><!-- wp:paragraph --><p>brdtertertreterewrew45</p><!-- /wp:paragraph --></div><!-- /wp:column --></div><!-- /wp:columns -->' },
    method: 'POST',
}).then(console.log);

This action is tottaly transparent to the user.

[jorgefilipecosta]

If we update some sidebar to use blocks, the sidebar_options structure is migrated in accordance with the RFC.
This causes some warnings on the existing widgets screen, these warnings may happen in production if the user downgrades to a previous WordPress version. On the same version even on the previous screen, this should not happen as we will implement functionality to handle the new structure even on the previous screen.

[jorgefilipecosta]

This PR was updated to be compatible with the current structure used by legacy widget block.
It is possible to test this integration by opening Gutenberg and pasting the following code in the console:

wp.apiFetch({
    path: '/__experimental/widget-areas/sidebar-1',
    method: 'GET',
}).then( ( { content } ) => window.wp.data.dispatch( 'core/editor' ).resetBlocks( window.wp.blocks.parse( content.raw ) ) );

It should load the sidebar widgets in the post editor using the legacy widgets block.

[noisysocks]

Thanks for picking this back up! I left some comments. Looking forward to chatting with you about all of this later today. I think the big thing we need to think through is how updating existing legacy widgets should work.

[noisysocks]

We ought to add unit tests for all of this.

[jorgefilipecosta]

Hi @noisysocks, thank you for your review 👍 Your feedback was applied!

[youknowriad]

I think these serialize functions shouldn't be "official" functions we support, As far as I understand, they are only needed for the legacy block behavior. They can be local to that or at least mark them as unstable.

[youknowriad]

I wonder if this should be marked experimental as well

[youknowriad]

What's blocking this @noisysocks @jorgefilipecosta I'd love to get a working prototype by the next Gutenberg release.

[jorgefilipecosta]

@youknowriad your comments were addressed. I'm not aware of any blocker and I think we just need the approval of the PR or additional feedback.

[youknowriad]

Let's move forward