Implement Input type=text UA Shadow DOM (#37065)

Implement Shadow Tree construction for input `type=text`, adding a text
control inner editor container and placeholder container. Subsequently,
due to the changes of the DOM tree structure, the changes will add a new
NodeFlag `IS_TEXT_CONTROL_INNER_EDITOR` to handle the following cases.
- If a mouse click button event hits a text control inner editor, it
will redirect the focus target to its shadow host.
- In text run's construction, the text control inner editor container
queries the selection from its shadow host. This is later used to
resolve caret and selection painting in the display list.

This will be the first step of fixing input `type=text` and other
single-line text input element widgets. Such as, implementing
`::placeholder` selector.



Testing: Existing WPT test and new Servo specific appearance WPT.
Fixes: #36307

---------

Signed-off-by: stevennovaryo <steven.novaryo@gmail.com>

components/script/dom/document.rs

@@ -1556,11 +1556,14 @@ impl Document {
                 return;
             }
 
+            // For a node within a text input UA shadow DOM, delegate the focus target into its shadow host.
+            // TODO: This focus delegation should be done with shadow DOM delegateFocus attribute.
+            let target_el = el.find_focusable_shadow_host_if_necessary();
+
             self.begin_focus_transaction();
-            // Try to focus `el`. If it's not focusable, focus the document
-            // instead.
+            // Try to focus `el`. If it's not focusable, focus the document instead.
             self.request_focus(None, FocusInitiator::Local, can_gc);
-            self.request_focus(Some(&*el), FocusInitiator::Local, can_gc);
+            self.request_focus(target_el.as_deref(), FocusInitiator::Local, can_gc);
         }
 
         let dom_event = DomRoot::upcast::<Event>(MouseEvent::for_platform_mouse_event(