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.
With this change, we now have ~1200 CellAllocators across both LibJS and
LibWeb in a normal WebContent instance.
This gives us a minimum heap size of 4.7 MiB in the scenario where we
only have one cell allocated per type. Of course, in practice there will
be many more of each type, so the effective overhead is quite a bit
smaller than that in practice.
I left a few types unconverted to this mechanism because I got tired of
doing this. :^)
The `<style>` element is allowed to be in the SVG namespace, so we now
support this element.
It has the same behaviour as the HTML namespace `<style>` element as
described in the spec.
"The semantics and processing of a ‘style’ and its attributes must be
the same as is defined for the HTML ‘style’ element."
