LibWeb: Parse CSS fit-content(<length-percentage>) values

Before this change, we only parsed fit-content as a standalone keyword,
but CSS-SIZING-3 added it as a function as well. I don't know of
anything else in CSS that is overloaded like this, so it ends up looking
a little awkward in the implementation.

Note that a lot of code had already been prepped for fit-content values
to have an argument, we just weren't parsing it.

Libraries/LibWeb/CSS/StyleValues/FitContentStyleValue.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2025, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSStyleValue.h>
+#include <LibWeb/CSS/PercentageOr.h>
+
+namespace Web::CSS {
+
+class FitContentStyleValue final : public CSSStyleValue {
+public:
+    static ValueComparingNonnullRefPtr<FitContentStyleValue> create()
+    {
+        return adopt_ref(*new (nothrow) FitContentStyleValue(LengthPercentage { Length::make_auto() }));
+    }
+    static ValueComparingNonnullRefPtr<FitContentStyleValue> create(LengthPercentage length_percentage)
+    {
+        return adopt_ref(*new (nothrow) FitContentStyleValue(move(length_percentage)));
+    }
+    virtual ~FitContentStyleValue() override = default;
+
+    virtual String to_string(SerializationMode) const override
+    {
+        if (m_length_percentage.is_auto())
+            return "fit-content"_string;
+        return MUST(String::formatted("fit-content({})", m_length_percentage.to_string()));
+    }
+
+    bool equals(CSSStyleValue const& other) const override
+    {
+        if (type() != other.type())
+            return false;
+        return m_length_percentage == other.as_fit_content().m_length_percentage;
+    }
+
+    [[nodiscard]] LengthPercentage const& length_percentage() const { return m_length_percentage; }
+
+private:
+    FitContentStyleValue(LengthPercentage length_percentage)
+        : CSSStyleValue(Type::FitContent)
+        , m_length_percentage(move(length_percentage))
+    {
+    }
+
+    LengthPercentage m_length_percentage;
+};
+
+}