LibWeb: Add ComputedCSSSstyleDeclaration and support 2 properties :^)

getComputedStyle(element) now returns a ComputedCSSStyleDeclaration
object, which is a live view of the computed style of a given element.

This works by ComputedCSSStyleDeclaration being a wrapper around an
element pointer. When you ask it for a CSS property, it gets the latest
computed style values from the element and returns them as a
CSS::StyleProperty object.

This first cut adds support for computed 'color' and 'display'.

In case the element doesn't have a corresponding node in the layout
tree, we fall back to using specified style instead. This is achieved by
performing an on-the-fly style resolution for the individual element and
then grabbing the requested property from that resolved style.

Userland/Libraries/LibWeb/CSS/ComputedCSSStyleDeclaration.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSStyleDeclaration.h>
+
+namespace Web::CSS {
+
+class ComputedCSSStyleDeclaration final : public CSSStyleDeclaration {
+public:
+    static NonnullRefPtr<ComputedCSSStyleDeclaration> create(DOM::Element& element)
+    {
+        return adopt_ref(*new ComputedCSSStyleDeclaration(element));
+    }
+
+    virtual ~ComputedCSSStyleDeclaration() override;
+
+    virtual size_t length() const override;
+    virtual String item(size_t index) const override;
+    virtual Optional<StyleProperty> property(PropertyID) const override;
+    virtual bool set_property(PropertyID, StringView css_text) override;
+
+private:
+    explicit ComputedCSSStyleDeclaration(DOM::Element&);
+
+    NonnullRefPtr<DOM::Element> m_element;
+};
+
+}