Rest API: add /revisions endpoint for global styles

[ramonopoly]

The following Global Revisions UI was added to Gutenberg in ​https://github.com/WordPress/gutenberg/pull/50089

To support this, an endpoint (/wp/v2/global-styles/revisions) that returns revisions for the global styles custom post was added and updated in:

​https://github.com/WordPress/gutenberg/pull/49974
​https://github.com/WordPress/gutenberg/pull/50234

We need to synchronize the PHP changes that add the endpoint to returns revisions for the global styles custom post.

The following Global Revisions UI was added to Gutenberg in:

• ​https://github.com/WordPress/gutenberg/pull/50089

To support this, an endpoint (/wp/v2/global-styles/revisions) that returns revisions for the global styles custom post was added and updated in:

• ​https://github.com/WordPress/gutenberg/pull/49974
• ​https://github.com/WordPress/gutenberg/pull/50234

This PR adds the endpoint to Core and adds tests.

Trac ticket: https://core.trac.wordpress.org/ticket/58524

## Testing

To test the endpoint in the site editor, fire up this branch and head over to the site editor.

In your browser's console, switch to the network tab and run the following :

await wp.apiFetch( { url: `/index.php?rest_route=%2Fwp%2Fv2%2Fglobal-styles%2F${ wp.data.select('core').__experimentalGetCurrentGlobalStylesId() }%2Frevisions&_locale=user` } );

npm run test:php -- --filter WP_REST_Global_Styles_Revisions_Controller_Test

Thank @TimothyBJacobs @spacedmonkey and @peterwilsoncc for helping me make this patch better.

I think I've addressed all of the feedback, (except for the ​context test) at least I think so. Please let me know where it can be improved.

A couple of nitpics. This is nearly there. Is there a committer / owner for this change?

I appreciate the continued feedback 🙇

I'll get those changes in today and will liaise with @tellthemachines to get it committed

[chaion07]

Thanks @ramonopoly for reporting this. We discussed this ticket during a recent bug-scrub session. Based on the feedback we are updating the ticket with the following changes:

1. Converting it into a Task
2. Keeping in the 6.3 milestone

Cheers!

Props to @audrasjb

[isabel_brison]

In 56082:

REST API: add revisions endpoint for global styles.

Adds an endpoint that returns revisions to the global styles custom post.

Props ramonopoly, peterwilsoncc, spacedmonkey, mukesh27, timothyblynjacobs.
Fixes #58524.

committed in r56082 / ​c345180.

Some missed formatting and also implementing feedback from ​https://github.com/WordPress/wordpress-develop/pull/4606#pullrequestreview-1500696132 (adding missing @covers to tests and swapping order of @ticket and @covers order)

Trac ticket: https://core.trac.wordpress.org/ticket/58524

The unit tests could do with @covers.

Thanks! PR for that here: ​https://github.com/WordPress/wordpress-develop/pull/4750

[isabel_brison]

In 56105:

REST API: updates test annotations for global styles revisions

Adds @covers annotations to tests and fixes incorrect spacing.

Props ramonopoly, mukesh27.
See #58524.

committed in r56105 / ​41277f1.

[audrasjb]

In 56272:

Docs: Fix indentation issue in WP_REST_Global_Styles_Revisions_Controller class.

Follow-up to [56082].

See #57840, #58524.

[joedolson]

In 56280:

Docs: Fix indentation issue in WP_REST_Global_Styles_Revisions_Controller class.

Follow-up to [56082].
Reviewed by joedolson.
Merges [56272] to the 6.3 branch.
See #57840, #58524.