Correctly merge custom CSS into global styles

[Mamaduka]

Update the get_block_editor_settings function and merge custom CSS directly into the $global_styles array.

Why?

Styles added directly to the $editor_settings['styles'] will get overridden later into the function (See L#440). This method only works in the Gutenberg plugin, where we use a filter to inject additional styles.

Testing Instructions

1. Using only 6.2 Beta 3, navigate to the Site Editor and add some custom CSS to the Heading block, for example, color: red
2. Note the Heading blocks are now red in the Site Editor.
3. Navigate to a page or post and add a Heading block. It is will not be red.
4. Now activate Gutenberg 15.2, and the Headings will be red.

Original report: ​https://github.com/WordPress/gutenberg/issues/48466

Gutenberg issue: ​https://github.com/WordPress/gutenberg/issues/48466
Trac ticket: https://core.trac.wordpress.org/ticket/57833

The test failure doesn't look related:

1) Tests_Admin_wpSiteHealth::test_object_cache_thresholds with data set #3 ('options_count', 100)
Failed asserting that false is true.

[poena]

I have followed the testing instructions and the PR solves the issue for me.

[sakibmd]

Thanks, @Mamaduka. PR looks good & it's working from my end.

The test passed after I re-run the failed job.

[gziolo]

In 55438:

Editor: Correctly merge custom CSS into global styles

Update the get_block_editor_settings function and merge custom CSS directly into the $global_styles array.

Fixes #57833.
Props Mamaduka, poena, sakibmd.

Committed with https://core.trac.wordpress.org/changeset/55438.