[TASK] Add SelectorComponent interface

This will be implemented by `CompoundSelector` and `Combinator` to break up
the components of a selector.

Part of #1325.

Author: JakeQZ

src/Property/Selector/SelectorComponent.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Property\Selector;
+
+interface SelectorComponent
+{
+    /**
+     * @return non-empty-string
+     */
+    public function getStringValue(): string;
+
+    /**
+     * @param non-empty-string $value
+     */
+    public function setStringValue(string $value): void;
+
+    /**
+     * @return int<0, max>
+     */
+    public function getSpecificity(): int;
+}