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.

---
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---------

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

tests/wpt/meta/MANIFEST.json

@@ -624655,6 +624655,13 @@
        {}
       ]
      ],
+     "Node-childNodes-cache-2.html": [
+      "9079fc6ea7a146cb854389bf4f1b8c302a102746",
+      [
+       null,
+       {}
+      ]
+     ],
      "Node-childNodes-cache.html": [
       "da9e32c6a9cf579f3d7c6ce2e3208e04f90d7105",
       [