LibWeb: Implement CSS Angle class

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

Userland/Libraries/LibWeb/CSS/Angle.h

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::CSS {
+
+class Angle {
+public:
+    enum class Type {
+        Calculated,
+        Deg,
+        Grad,
+        Rad,
+        Turn,
+    };
+
+    static Optional<Type> unit_from_name(StringView);
+
+    Angle(int value, Type type);
+    Angle(float value, Type type);
+    static Angle make_calculated(NonnullRefPtr<CalculatedStyleValue>);
+    static Angle make_degrees(float);
+    Angle percentage_of(Percentage const&) const;
+
+    bool is_calculated() const { return m_type == Type::Calculated; }
+
+    String to_string() const;
+    float to_degrees() const;
+
+    bool operator==(Angle const& other) const
+    {
+        if (is_calculated())
+            return m_calculated_style == other.m_calculated_style;
+        return m_type == other.m_type && m_value == other.m_value;
+    }
+
+    bool operator!=(Angle const& other) const
+    {
+        return !(*this == other);
+    }
+
+private:
+    StringView unit_name() const;
+
+    Type m_type;
+    float m_value { 0 };
+    RefPtr<CalculatedStyleValue> m_calculated_style;
+};
+
+}