Exclude CDATASection nodes from Node::normalize. I made it under the
assumption that CDATAs can't have children so we don't need to go into
the `else node.Normalize()` branch on line 3485 with them, hope this is
correct.
Testing: covered by `dom/nodes/Node-normalize.html`
Fixes: https://github.com/servo/servo/issues/37006
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Signed-off-by: Rodion Borovyk <rodion.borovyk@gmail.com>