Implement shadow dom slots (#35013)

* Implement slot-related algorithms

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

* Hook up slot elements to DOM creation logic

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

* Set a slot assignment mode for servo-internal shadow roots

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

* Assign slots when a slottable's slot attribute changes

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

* Properly compute slot name

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

* ./mach test-tidy

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

* Update <slot> name when name attribute changes

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

* Implement fast path for Node::assign_slottables_for_a_tree

assign_slottables_for_a_tree traverses all descendants of the node
and is potentially very expensive. If the node is not a shadow root
then assigning slottables to it won't have any effect, so we
take a fast path out.

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

* Move slottable data into ElementRareData

This shrinks all element descendants back to their
original size.

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

* Address review comments

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

* Update WPT expectations

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

---------

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

components/script/dom/mod.rs

@@ -398,6 +398,7 @@ pub(crate) mod htmlquoteelement;
 #[allow(dead_code)]
 pub(crate) mod htmlscriptelement;
 pub(crate) mod htmlselectelement;
+pub(crate) mod htmlslotelement;
 pub(crate) mod htmlsourceelement;
 pub(crate) mod htmlspanelement;
 pub(crate) mod htmlstyleelement;