Fix inconsistency of block style variation names causing duplicate UI options

[aaronrobertshaw]

This ticket tracks the backporting of PHP files for the following Gutenberg fix:

​https://github.com/WordPress/gutenberg/pull/62550

These changes prevent an incorrect duplicate block style from appearing within the block styles panel due to inconsistencies between variation names due to case sensitivity.

This fix then also means that variation definitions from different sources get merged and applied correctly.

Backports the PHP changes from ​https://github.com/WordPress/gutenberg/pull/62550 fixing an issue with duplicate block style options appearing and variation definitions from different sources not merging correctly.

Test Instructions:

1. Register a block style variation via register_block_style snippet below

   register_block_style(
                           'core/group',
                           array(
                                   'name'         => 'MyVariation',
                                   'label'        => __( 'My variation', 'twentytwentyfour' ),
                                   'inline_style' => '
                                   .is-style-MyVariation {
                                           background-color: red;
                                   }',
                           )
                   );
   

2. Create theme.json partial file within /styles/partial.json using the following snippet

   {
       "$schema": "https://schemas.wp.org/trunk/theme.json",
       "version": 2,
       "title": "My variation",
       "slug": "MyVariation",
       "blockTypes": [ "core/group" ],
       "styles": {
               "color": {
                       "background": "blue",
                       "text": "white"
               }
       }
   }
   

3. Open the site editor, select a group block and confirm only a single block style option other than default
4. Select that block style and confirm that the partial theme.json files' style applies correctly

Trac ticket: https://core.trac.wordpress.org/ticket/61440

[oandregal]

In 58413:

Section styles: add slug to override non-kebab-cased variations.

Props aaronrobertshaw, oandregal.
Fixes #61440.

Committed at https://core.trac.wordpress.org/changeset/58413