LibWeb: Make FormattingContext::run() take available space as input

Instead of formatting contexts flailing around to figure out from the
"inside" how much space is available on the "outside", we should
provide the amount of available space in both axes as an input to run().

This basically means that when something creates a nested formatting
context, the parent context is responsible for telling the nested context
how much space is available for layout. This information is provided
immediately when invoking run().

Note that this commit doesn't pass accurate values in all cases yet.
This first step just makes it build, and passes available values in some
cases where getting them was trivial.

Userland/Libraries/LibWeb/Layout/AvailableSpace.cpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Layout/AvailableSpace.h>
+#include <math.h>
+
+namespace Web::Layout {
+
+AvailableSpace AvailableSpace::make_definite(float value)
+{
+    return AvailableSpace { Type::Definite, value };
+}
+
+AvailableSpace AvailableSpace::make_indefinite()
+{
+    return AvailableSpace { Type::Indefinite, INFINITY };
+}
+
+AvailableSpace AvailableSpace::make_min_content()
+{
+    return AvailableSpace { Type::MinContent, 0 };
+}
+
+AvailableSpace AvailableSpace::make_max_content()
+{
+    return AvailableSpace { Type::MaxContent, INFINITY };
+}
+
+String AvailableSpace::to_string() const
+{
+    switch (m_type) {
+    case Type::Definite:
+        return String::formatted("definite({})", m_value);
+    case Type::Indefinite:
+        return "indefinite";
+    case Type::MinContent:
+        return "min-content";
+    case Type::MaxContent:
+        return "max-content";
+    }
+    VERIFY_NOT_REACHED();
+}
+
+AvailableSpace::AvailableSpace(Type type, float value)
+    : m_type(type)
+    , m_value(value)
+{
+}
+
+}