LibWeb: Parse descriptors as style values, using the JSON data

The goal here is to do something a bit smarter with the parsing here
than we do for properties. Instead of the JSON saying "here are the
values, and here are the keywords, and we can have up to 3", here we
place the syntax in the JSON directly (though currently broken up as
one string per option) and then we attempt to parse each one in
sequence. It's something we'll need eventually for `@property` among
other things.

...However, in this first pass, I've gone with the simplest option of
hard-coding the types instead of figuring them out properly. So there's
a PositivePercentage type and a UnicodeRangeTokens type, instead of
properly implementing the grammar for those in a generic way.

Libraries/LibWeb/CSS/Descriptor.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibWeb/CSS/DescriptorID.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::CSS {
+
+struct Descriptor {
+    ~Descriptor();
+
+    DescriptorID descriptor_id;
+    NonnullRefPtr<CSSStyleValue const> value;
+};
+
+}