LibWeb: Add method for listing all style sheets on a page

This will be used by the inspector, for showing style sheet contents.

Identifying a specific style sheet is a bit tricky. Depending on where
it came from, a style sheet may have a URL, it might be associated with
a DOM element, both, or neither. This varied information is wrapped in
a new StyleSheetIdentifier struct.

Userland/Libraries/LibWeb/CSS/StyleSheetIdentifier.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "StyleSheetIdentifier.h"
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+
+namespace Web::CSS {
+
+StringView style_sheet_identifier_type_to_string(StyleSheetIdentifier::Type type)
+{
+    switch (type) {
+    case StyleSheetIdentifier::Type::StyleElement:
+        return "StyleElement"sv;
+    case StyleSheetIdentifier::Type::LinkElement:
+        return "LinkElement"sv;
+    case StyleSheetIdentifier::Type::ImportRule:
+        return "ImportRule"sv;
+    case StyleSheetIdentifier::Type::UserAgent:
+        return "UserAgent"sv;
+    case StyleSheetIdentifier::Type::UserStyle:
+        return "UserStyle"sv;
+    }
+    VERIFY_NOT_REACHED();
+}
+
+Optional<StyleSheetIdentifier::Type> style_sheet_identifier_type_from_string(StringView string)
+{
+    if (string == "StyleElement"sv)
+        return StyleSheetIdentifier::Type::StyleElement;
+    if (string == "LinkElement"sv)
+        return StyleSheetIdentifier::Type::LinkElement;
+    if (string == "ImportRule"sv)
+        return StyleSheetIdentifier::Type::ImportRule;
+    if (string == "UserAgent"sv)
+        return StyleSheetIdentifier::Type::UserAgent;
+    if (string == "UserStyle"sv)
+        return StyleSheetIdentifier::Type::UserStyle;
+    return {};
+}
+
+}
+
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder& encoder, Web::CSS::StyleSheetIdentifier const& style_sheet_source)
+{
+    TRY(encoder.encode(style_sheet_source.type));
+    TRY(encoder.encode(style_sheet_source.dom_element_unique_id));
+    TRY(encoder.encode(style_sheet_source.url));
+
+    return {};
+}
+
+template<>
+ErrorOr<Web::CSS::StyleSheetIdentifier> decode(Decoder& decoder)
+{
+    auto type = TRY(decoder.decode<Web::CSS::StyleSheetIdentifier::Type>());
+    auto dom_element_unique_id = TRY(decoder.decode<Optional<i32>>());
+    auto url = TRY(decoder.decode<Optional<String>>());
+
+    return Web::CSS::StyleSheetIdentifier {
+        .type = type,
+        .dom_element_unique_id = move(dom_element_unique_id),
+        .url = move(url),
+    };
+}
+
+}