chore: make classes readonly+final where applicable

All classes are now final + readonly, except `MediaTypeParserException` which is only final since `Exception` is not readonly.

src/MediaType.php

@@ -9,24 +9,24 @@
  * @see https://mimesniff.spec.whatwg.org/
  * @license MIT
  */
-class MediaType implements Stringable {
+final readonly class MediaType implements Stringable {
 	/**
 	 * @see https://mimesniff.spec.whatwg.org/#type
 	 * @var string
 	 */
-	private $type;
+	private string $type;
 
 	/**
 	 * @see https://mimesniff.spec.whatwg.org/#subtype
 	 * @var string
 	 */
-	private $subType;
+	private string $subType;
 
 	/**
 	 * @see https://mimesniff.spec.whatwg.org/#parameters
 	 * @var string[]
 	 */
-	private $parameters;
+	private array $parameters;
 
 	/**
 	 * @param string $type

src/MediaTypeParser.php

@@ -8,7 +8,7 @@
 /**
  * @see https://mimesniff.spec.whatwg.org/#parsing-a-mime-type
  */
-class MediaTypeParser {
+final readonly class MediaTypeParser {
 	public function parseOrNull( string $s ): MediaType|null {
 		try {
 			return $this->parse( $s );

src/MediaTypeParserException.php

@@ -7,5 +7,5 @@
 /**
  * @license MIT
  */
-class MediaTypeParserException extends Exception {
+final class MediaTypeParserException extends Exception {
 }

src/Utf8Utils.php

@@ -9,7 +9,7 @@
  * @internal
  * @license MIT
  */
-class Utf8Utils {
+final readonly class Utf8Utils {
 	public static function onlyContains( string $input, callable $predicateFn ): bool {
 		if ( $input === '' ) {
 			return true;