Rewrite node insertion algorithm to match the spec (#35999)

Per [spec](https://dom.spec.whatwg.org/#concept-node-insert), adoption
of new node should be done while inserting the node. This patch moves
the call site of `adopt` to inside `insert` to match it.

It also rewrites some existing code to better match the spec without any
behavioral changes.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox
is checked, so that we can help you if you get stuck somewhere along the
way.-->

<!-- Pull requests that do not address these steps are welcome, but they
will require additional verification as part of the review process. -->

---------

Signed-off-by: Xiaocheng Hu <xiaochengh.work@gmail.com>

tests/wpt/meta/html/semantics/document-metadata/the-style-element/mutations.window.js.ini

@@ -1,6 +1,3 @@
 [mutations.window.html]
   [Mutating the style element: mutating a Comment node]
     expected: FAIL
-
-  [Mutating the style element: inserting an empty DocumentFragment node]
-    expected: FAIL