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/element.rs

@@ -1662,6 +1662,27 @@ impl Element {
         )
     }
 
+    /// Returns the focusable shadow host if this is a text control inner editor.
+    /// This is a workaround for the focus delegation of shadow DOM and should be
+    /// used only to delegate focusable inner editor of [HTMLInputElement] and
+    /// [HTMLTextAreaElement].
+    pub(crate) fn find_focusable_shadow_host_if_necessary(&self) -> Option<DomRoot<Element>> {
+        if self.is_focusable_area() {
+            Some(DomRoot::from_ref(self))
+        } else if self.upcast::<Node>().is_text_control_inner_editor() {
+            let containing_shadow_host = self.containing_shadow_root().map(|root| root.Host());
+            assert!(
+                containing_shadow_host
+                    .as_ref()
+                    .is_some_and(|e| e.is_focusable_area()),
+                "Containing shadow host is not focusable"
+            );
+            containing_shadow_host
+        } else {
+            None
+        }
+    }
+
     pub(crate) fn is_actually_disabled(&self) -> bool {
         let node = self.upcast::<Node>();
         match node.type_id() {