LibWeb: Add CSS::Size to represent the full range of CSS size values

Until now, we've been using CSS::LengthPercentage, sometimes wrapped in
Optional, to represent CSS sizes.

This meant we could not support modern values like `min-content`,
`max-content`, `fit-content(<length>)`. We were also conflating `none`
and `auto` which made the `min-*` and `max-*` properties confusing.

The new CSS::Size class covers all possible size values as individual
substates. It'll be quite a bit of work to make all layout code aware of
the additional features, this patch merely makes the new type available.

Userland/Libraries/LibWeb/CSS/Size.cpp

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/Length.h>
+#include <LibWeb/CSS/Size.h>
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+Size::Size(Type type, LengthPercentage length_percentage)
+    : m_type(type)
+    , m_length_percentage(move(length_percentage))
+{
+}
+
+CSS::Length Size::resolved(Layout::Node const& node, Length const& reference_value) const
+{
+    return m_length_percentage.resolved(node, reference_value);
+}
+
+Size Size::make_auto()
+{
+    return Size { Type::Auto, Length::make_auto() };
+}
+
+Size Size::make_length(Length length)
+{
+    return Size { Type::Length, move(length) };
+}
+
+Size Size::make_percentage(Percentage percentage)
+{
+    return Size { Type::Percentage, move(percentage) };
+}
+
+Size Size::make_min_content()
+{
+    return Size { Type::MinContent, Length::make_auto() };
+}
+
+Size Size::make_max_content()
+{
+    return Size { Type::MaxContent, Length::make_auto() };
+}
+
+Size Size::make_fit_content(Length available_space)
+{
+    return Size { Type::FitContent, move(available_space) };
+}
+
+Size Size::make_none()
+{
+    return Size { Type::None, Length::make_auto() };
+}
+
+bool Size::contains_percentage() const
+{
+    switch (m_type) {
+    case Type::Auto:
+    case Type::MinContent:
+    case Type::MaxContent:
+    case Type::None:
+        return false;
+    default:
+        return m_length_percentage.contains_percentage();
+    }
+}
+
+}