Added @layer directive to list of nested directives.

Updated `directive::parse()` to set a new `empty` property, for detecting when a directive has curly brackets, but no content, enabling directives with just properties not to be deleted.
Added tests.
Updated dependencies.

.gitignore

@@ -5,3 +5,4 @@
 /vendor/
 /coverage
 /.phpunit.cache
+/.phpunit.result.cache

src/config.php

@@ -8,7 +8,7 @@ class config {
 	 * @var array<array> $config Object configuration array
 	 */
 	protected array $config = [
-		'nested' => ['@media', '@supports', '@keyframes', '@-webkit-keyframes', '@-moz-keyframes', '@-o-keyframes', '@document', '@-moz-document', '@container'], // directive that can have nested rules
+		'nested' => ['@media', '@supports', '@keyframes', '@-webkit-keyframes', '@-moz-keyframes', '@-o-keyframes', '@document', '@-moz-document', '@container', '@layer'], // directive that can have nested rules
 		'spaced' => ['calc', 'min', 'max', 'clamp'], // values where spaces between operators must be retained
 		'quoted' => ['content', 'format', 'counters', '@charset', 'syntax', 'font-feature-settings', '-webkit-font-feature-settings', '-moz-font-feature-settings', 'quotes', 'text-overflow'], // directives or properties where the contained values must be quoted
 		'casesensitive' => ['url'], // property values that should not be lowercased