Fix: Slot Assignment mode according to the spec (#36620)

When parsing a Declarative Shadow DOM, the slot assignment isn't being
triggered as mentioned in the
[spec](https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead:~:text=Attach%20a%20shadow%20root%20with%20declarative%20shadow%20host%20element%2C%20mode%2C%20clonable%2C%20serializable%2C%20delegatesFocus%2C%20%22named%22%2C%20and%20registry.).

When creating a shadow root declaratively, SlotAssignment mode was set
to "manual" instead of "named".

Tests: The followings should get fully passed with this change, so
`.ini`s are removed.
 - `declarative-shadow-dom-basic.html` 
 - `declarative-shadow-dom-write-to-iframe.html`
 - `declarative-shadow-dom-repeats.html`

Fixes: #36100

---------

Signed-off-by: saku <saku@email.sakupi01.com>

components/script/dom/servoparser/mod.rs

@@ -1462,7 +1462,7 @@ impl TreeSink for Sink {
             clonable,
             serializable,
             delegatesfocus,
-            SlotAssignmentMode::Manual,
+            SlotAssignmentMode::Named,
             CanGc::note(),
         ) {
             Ok(shadow_root) => {

tests/wpt/meta/shadow-dom/declarative/declarative-shadow-dom-basic.html.ini

@@ -1,6 +0,0 @@
-[declarative-shadow-dom-basic.html]
-  [Declarative Shadow DOM: Basic test]
-    expected: FAIL
-
-  [Declarative Shadow DOM: Fragment parser basic test]
-    expected: FAIL

tests/wpt/meta/shadow-dom/declarative/declarative-shadow-dom-repeats.html.ini

@@ -1,3 +0,0 @@
-[declarative-shadow-dom-repeats.html]
-  [Calling attachShadow() on declarative shadow root must match all parameters]
-    expected: FAIL

tests/wpt/meta/shadow-dom/declarative/declarative-shadow-dom-write-to-iframe.html.ini

@@ -1,3 +0,0 @@
-[declarative-shadow-dom-write-to-iframe.html]
-  [`document.write` on inner iframe handles declarative shadow DOM]
-    expected: FAIL