Twenty Twenty-One: Clean up Button styles.

This change restructures the Button element styles for better a11y and expected color behaviors. Here what has changed:

- The `button-style()` mixin is now the ''single source or truth'' for how Button should look site-wide.
  - Button Block, File Block, and Search form Blocks all rely on this one mixin.
  - The same styles is also applied to the `<button>` element which appears in widgets, the 404 search form and comments form.
- Improves expected button styles for various conditions and contexts as follows: 
  - User color palette selections for Buttons are retained regardless of a parent block’s color settings or dark-mode.
  - Supports both Default/Filled styles and Outline styles.
  - More consistent `:hover` and `:active` styles.
  - `:focus` styles are now always visible and legible.
- Adds a `--local-color` color variable to scope color relationships to nested blocks.
- Reduces selectors in `style.css` output.
- 1:1 experience between editor and front end button styles.
- Properly supports Dark-mode.

Props allancole, poena, scruffian, megphillips91.
Fixes #51927.
Built from https://develop.svn.wordpress.org/trunk@49987


git-svn-id: http://core.svn.wordpress.org/trunk@49688 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49986';
+$wp_version = '5.7-alpha-49987';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.