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

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>

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