LibWeb: Implement CSS Resolution class

This corresponds to `<resolution>` in the grammar.

Userland/Libraries/LibWeb/CSS/Resolution.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::CSS {
+
+class Resolution {
+public:
+    enum class Type {
+        Dpi,
+        Dpcm,
+        Dppx,
+    };
+
+    static Optional<Type> unit_from_name(StringView);
+
+    Resolution(int value, Type type);
+    Resolution(float value, Type type);
+
+    String to_string() const;
+    float to_dots_per_pixel() const;
+
+    bool operator==(Resolution const& other) const
+    {
+        return m_type == other.m_type && m_value == other.m_value;
+    }
+
+    bool operator!=(Resolution const& other) const
+    {
+        return !(*this == other);
+    }
+
+private:
+    StringView unit_name() const;
+
+    Type m_type;
+    float m_value { 0 };
+};
+}