style: Part 2 - Add page-size attribute to nsStyleStruct and property parsing

This parsing is hidden behind the pref layout.css.page-size.enabled.

It isn't ideal that we parse this as a property, but we can't treat it as a
descriptor because of compatibility issues with other browsers. There are also
outstanding spec issues related to how descriptors like page-size are cascaded,
and whether the !important specifier is valid or not.

Differential Revision: https://phabricator.services.mozilla.com/D103958

components/style/properties/gecko.mako.rs

@@ -781,6 +781,9 @@ fn static_assert() {
     % endfor
 </%self:impl_trait>
 
+<%self:impl_trait style_struct_name="Page">
+</%self:impl_trait>
+
 <% skip_position_longhands = " ".join(x.ident for x in SIDES) %>
 <%self:impl_trait style_struct_name="Position"
                   skip_longhands="${skip_position_longhands}