LibWeb: Add a new code generator for CSS enums

Alias values are represented by "alias-name=real-name".

We have a lot of repetitive code for converting between ValueID and
property-specific enums. Let's see if we can generate it. :^)

This first step just produces the enums, from a JSON file. The values in
there are a duplication of what's in Properties.json, but eventually
those will go away.

Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSEnums.cpp

@@ -0,0 +1,100 @@
+/*
+ * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "GeneratorUtil.h"
+#include <AK/SourceGenerator.h>
+#include <AK/StringBuilder.h>
+#include <LibCore/ArgsParser.h>
+#include <LibMain/Main.h>
+
+ErrorOr<void> generate_header_file(JsonObject& enums_data, Core::Stream::File& file);
+ErrorOr<void> generate_implementation_file(JsonObject& enums_data, Core::Stream::File& file);
+
+ErrorOr<int> serenity_main(Main::Arguments arguments)
+{
+    StringView generated_header_path;
+    StringView generated_implementation_path;
+    StringView identifiers_json_path;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(generated_header_path, "Path to the Enums header file to generate", "generated-header-path", 'h', "generated-header-path");
+    args_parser.add_option(generated_implementation_path, "Path to the Enums implementation file to generate", "generated-implementation-path", 'c', "generated-implementation-path");
+    args_parser.add_option(identifiers_json_path, "Path to the JSON file to read from", "json-path", 'j', "json-path");
+    args_parser.parse(arguments);
+
+    auto json = TRY(read_entire_file_as_json(identifiers_json_path));
+    VERIFY(json.is_object());
+    auto enums_data = json.as_object();
+
+    auto generated_header_file = TRY(Core::Stream::File::open(generated_header_path, Core::Stream::OpenMode::Write));
+    auto generated_implementation_file = TRY(Core::Stream::File::open(generated_implementation_path, Core::Stream::OpenMode::Write));
+
+    TRY(generate_header_file(enums_data, *generated_header_file));
+    TRY(generate_implementation_file(enums_data, *generated_implementation_file));
+
+    return 0;
+}
+
+ErrorOr<void> generate_header_file(JsonObject& enums_data, Core::Stream::File& file)
+{
+    StringBuilder builder;
+    SourceGenerator generator { builder };
+
+    generator.append(R"~~~(
+#pragma once
+
+namespace Web::CSS {
+
+enum class ValueID;
+
+)~~~");
+
+    enums_data.for_each_member([&](auto& name, auto& value) {
+        VERIFY(value.is_array());
+        auto& members = value.as_array();
+
+        auto enum_generator = generator.fork();
+        enum_generator.set("name:titlecase", title_casify(name));
+        enum_generator.appendln("enum class @name:titlecase@ {");
+
+        for (auto& member : members.values()) {
+            auto member_name = member.to_string();
+            // Don't include aliases in the enum.
+            if (member_name.contains('='))
+                continue;
+            auto member_generator = enum_generator.fork();
+            member_generator.set("member:titlecase", title_casify(member_name));
+            member_generator.appendln("    @member:titlecase@,");
+        }
+
+        enum_generator.appendln("};\n");
+    });
+
+    generator.appendln("}");
+
+    TRY(file.write(generator.as_string_view().bytes()));
+    return {};
+}
+
+ErrorOr<void> generate_implementation_file(JsonObject& enums_data, Core::Stream::File& file)
+{
+    StringBuilder builder;
+    SourceGenerator generator { builder };
+
+    generator.append(R"~~~(
+#include <LibWeb/CSS/Enums.h>
+
+namespace Web::CSS {
+)~~~");
+
+    // TODO: Generate some things!
+    (void)enums_data;
+
+    generator.appendln("}");
+
+    TRY(file.write(generator.as_string_view().bytes()));
+    return {};
+}