Support exporting shadow parts with the exportparts attribute (#37345)

The attribute is implemented as a new `AttrValue` variant containing the
mappings of exported part names
(https://github.com/servo/stylo/pull/197).

Take a look at the [MDN
page](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Global_attributes/exportparts)
for more information about the attribute.


Testing: Covered by WPT
Fixes: https://github.com/servo/servo/issues/35349

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/script/dom/element.rs

@@ -4226,6 +4226,7 @@ impl VirtualMethods for Element {
             local_name!("class") | local_name!("part") => {
                 AttrValue::from_serialized_tokenlist(value.into())
             },
+            local_name!("exportparts") => AttrValue::from_shadow_parts(value.into()),
             _ => self
                 .super_type()
                 .unwrap()