LibWeb/CSS: Move PageSelector into its own files

The main motivation here is that the CSS Parser needs to know about
PageSelectorList so that we can parse one in
`CSSPageRule::set_selector_text()`. Including all of `CSSPageRule.h`
there would pull in a lot of other headers that aren't needed.

Libraries/LibWeb/CSS/PageSelector.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/PageSelector.h>
+
+namespace Web::CSS {
+
+Optional<PagePseudoClass> page_pseudo_class_from_string(StringView input)
+{
+    if (input.equals_ignoring_ascii_case("blank"sv))
+        return PagePseudoClass::Blank;
+    if (input.equals_ignoring_ascii_case("first"sv))
+        return PagePseudoClass::First;
+    if (input.equals_ignoring_ascii_case("left"sv))
+        return PagePseudoClass::Left;
+    if (input.equals_ignoring_ascii_case("right"sv))
+        return PagePseudoClass::Right;
+    return {};
+}
+
+StringView to_string(PagePseudoClass pseudo_class)
+{
+    switch (pseudo_class) {
+    case PagePseudoClass::Blank:
+        return "blank"sv;
+    case PagePseudoClass::First:
+        return "first"sv;
+    case PagePseudoClass::Left:
+        return "left"sv;
+    case PagePseudoClass::Right:
+        return "right"sv;
+    }
+    VERIFY_NOT_REACHED();
+}
+
+PageSelector::PageSelector(Optional<FlyString> name, Vector<PagePseudoClass> pseudo_classes)
+    : m_name(move(name))
+    , m_pseudo_classes(move(pseudo_classes))
+{
+}
+
+String PageSelector::serialize() const
+{
+    StringBuilder builder;
+    if (m_name.has_value())
+        builder.append(m_name.value());
+    for (auto pseudo_class : m_pseudo_classes)
+        builder.appendff(":{}", to_string(pseudo_class));
+    return builder.to_string_without_validation();
+}
+
+}