Global styles: preload `/global-styles` endpoint responses [regression]

[ramonopoly]

A PR to track the backporting of ​https://github.com/WordPress/gutenberg/pull/66468

This fixes a regression whereby requests to global styles endpoints were not being preloaded, resulting in several requests being fired clientside unnecessarily.

For performance reasons, we should preload the requests so that the data is in the store and ready to use straight away.

The outcome is that the editor loads more quickly.

A PR to track the backporting of ​https://github.com/WordPress/gutenberg/pull/66468

This fixes a regression whereby requests to global styles endpoints were not being preloaded, resulting in several requests being fired clientside unnecessarily.

For performance reasons, we should preload the requests so that the data is in the store and ready to use straight away.

The outcome is that the editor loads more quickly.

### Testing

Fire up this branch and head to the site editor.

Check that global-styles endpoints aren't fetched client side, but are, rather, preloaded via ​rest_preload_api_request

Do the same for the post editor.

Trac ticket: https://core.trac.wordpress.org/ticket/62315

[ramonopoly]

In 59316:

Global styles: preload /global-styles endpoint responses

This commit fixes a regression whereby requests to global styles endpoints were not being preloaded, resulting in several requests being fired clientside unnecessarily.

For performance reasons, we preload the requests so that the data is in the editor store and ready to use straight away.

The outcome is that the editor loads more quickly.

Follow-up to [62042].

Props ellatrix, ramonopoly, apermo, peterwilsoncc.
Fixes #62315.

Committed in r59316 / ​dc4bd050

[ramonopoly]

Reopening so that this regression fix can be considered for backporting to 6.7.
Thank you!

[peterwilsoncc]

Approved for merge to the 6.7 branch.

[peterwilsoncc]

In 59324:

Global styles: preload /global-styles endpoint responses

This commit fixes a regression whereby requests to global styles endpoints were not being preloaded, resulting in several requests being fired clientside unnecessarily.

For performance reasons, we preload the requests so that the data is in the editor store and ready to use straight away.

The outcome is that the editor loads more quickly.

Follow-up to [62042].

Reviewed by peterwilsoncc.
Merges [59316] to the 6.7 branch.

Props ellatrix, ramonopoly, apermo, peterwilsoncc.
Fixes #62315.