Themes: Add opt-in Appearance Tools support for Classic Themes.

This changeset merges the following changes from Gutenberg repository: 

- Allow themes without `theme.json` to opt-in to appearance tools via `add_theme_support( 'appearance-tools' );`
- Update `wpThemeJsonResolver` unit tests accordingly

See the following pull request for more info: https://github.com/WordPress/gutenberg/pull/43337

Props ironprogrammer, audrasjb.
Fixes #57460.


git-svn-id: https://develop.svn.wordpress.org/trunk@55067 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json-resolver.php

@@ -322,6 +322,11 @@ class WP_Theme_JSON_Resolver {
 
 			// Classic themes without a theme.json don't support global duotone.
 			$theme_support_data['settings']['color']['defaultDuotone'] = false;
+
+			// Allow themes to enable appearance tools via theme_support.
+			if ( current_theme_supports( 'appearance-tools' ) ) {
+				$theme_support_data['settings']['appearanceTools'] = true;
+			}
 		}
 		$with_theme_supports = new WP_Theme_JSON( $theme_support_data );
 		$with_theme_supports->merge( static::$theme );