Block Patterns: Also load patterns from bare HTML files

Alternative take based on #36751 (comment)

lib/block-patterns.php

@@ -322,6 +322,61 @@ function _register_theme_block_patterns() {
 	// if applicable.
 	foreach ( wp_get_active_and_valid_themes() as $theme ) {
 		$dirpath = $theme . '/patterns/';
+		if ( file_exists( $dirpath ) ) {
+			$files = glob( $dirpath . '*.html' );
+			if ( $files ) {
+				foreach ( $files as $file ) {
+					// Parse pattern slug from file name.
+					if ( ! preg_match( '#/(?P<slug>[A-z0-9_-]+)\.html$#', $file, $matches ) ) {
+						continue; // FIXME: Consider logging notice.
+					}
+					// Example name: twentytwentytwo/query-grid-posts.
+					$pattern_name = get_stylesheet() . '/' . $matches['slug'];
+
+					$pattern_data = get_file_data( $file, $default_headers );
+
+					// Title is a required property.
+					if ( ! $pattern_data['title'] ) {
+						continue;
+					}
+
+					// For properties of type array, parse data as comma-separated.
+					foreach ( array( 'categories', 'keywords', 'blockTypes' ) as $property ) {
+						$pattern_data[ $property ] = array_filter(
+							preg_split(
+								'/[\s,]+/',
+								(string) $pattern_data[ $property ]
+							)
+						);
+					}
+
+					// Parse properties of type int.
+					foreach ( array( 'viewportWidth' ) as $property ) {
+						$pattern_data[ $property ] = (int) $pattern_data[ $property ];
+					}
+
+					// Remove up empty values, so as not to override defaults.
+					foreach ( array_keys( $default_headers ) as $property ) {
+						if ( empty( $pattern_data[ $property ] ) ) {
+							unset( $pattern_data[ $property ] );
+						}
+					}
+
+					// The actual pattern is everything following the leading comment.
+					$raw_content             = file_get_contents( $file );
+					$token                   = '-->';
+					$pattern_data['content'] = substr(
+						$raw_content,
+						strpos( $raw_content, $token ) + strlen( $token )
+					);
+					if ( ! $pattern_data['content'] ) {
+						continue;
+					}
+
+					register_block_pattern( $pattern_name, $pattern_data );
+				}
+			}
+		}
 		if ( file_exists( $dirpath ) ) {
 			$files = glob( $dirpath . '*.php' );
 			if ( $files ) {