[TASK] Add SelectorComponent interface (#1478)

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 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Property\Selector;
+
+/**
+ * This interface is for a class that represents a part of a selector which is either a compound selector (or a simple
+ * selector, which is effectively a compound selector without any compounding) or a selector combinator.
+ *
+ * It allows a selector to be represented as an array of objects that implement this interface.
+ * This is the formal definition:
+ * selector = compound-selector [combinator, compound-selector]*
+ *
+ * The selector is comprised of an array of alternating types that can't be easily represented in a type-safe manner
+ * without this.
+ *
+ * 'Selector component' is not a known grammar in the spec, but a convenience for the implementation.
+ *
+ * @see https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Selectors/Selector_structure
+ * @see https://www.w3.org/TR/selectors-4/#structure
+ */
+interface SelectorComponent
+{
+    /**
+     * @return non-empty-string
+     */
+    public function getValue(): string;
+
+    /**
+     * @param non-empty-string $value
+     */
+    public function setValue(string $value): void;
+
+    /**
+     * @return int<0, max>
+     */
+    public function getSpecificity(): int;
+}