LibWeb: Implement IntegerStyleValue, for holding <integer>

Having one StyleValue for `<number>` and `<integer>` is making user code
more complicated than it needs to be. We know based on the property
being parsed, whether it wants a `<number>` or an `<integer>`, so we
can use separate StyleValue types for these.

Userland/Libraries/LibWeb/CSS/StyleValues/IntegerStyleValue.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+class IntegerStyleValue : public StyleValueWithDefaultOperators<IntegerStyleValue> {
+public:
+    static ErrorOr<ValueComparingNonnullRefPtr<IntegerStyleValue>> create(i64 value)
+    {
+        return adopt_nonnull_ref_or_enomem(new (nothrow) IntegerStyleValue(value));
+    }
+
+    i64 integer() const { return m_value; }
+
+    virtual ErrorOr<String> to_string() const override;
+
+    bool properties_equal(IntegerStyleValue const& other) const { return m_value == other.m_value; }
+
+private:
+    explicit IntegerStyleValue(i64 value)
+        : StyleValueWithDefaultOperators(Type::Integer)
+        , m_value(value)
+    {
+    }
+
+    i64 m_value { 0 };
+};
+
+}