In DOM APIs, `Node.childNodes` is a `NodeList` that has an
`.item(index)` method that allows random access, but DOM nodes only
store pointers to their first/last child and next/previous sibling. The
previous implementation involved keeping a "last accessed" pointer, and
a significant amount of logic to find the requested index by walking
next/previous sibling pointers from whichever of last accessed, first
child, or last child is nearest. This logic sometimes incorrectly
assumed the (nullable) last accessed pointer to be present, causing a
panic in `Option::unwrap`.
Rather than try to fix that logic, this replaces entirely with the
approach suggested in #25206 and used [by
Firefox](https://searchfox.org/firefox-main/source/dom/base/nsChildContentList.h):
keep a cached `Vec` of pointers to all child nodes, created lazily when
needed, and invalidated whenever any part of it changes.
Testing: the first commit adds a failing WPT crashtest, the second
commit fixes it
Fixes: https://github.com/servo/servo/issues/25206
Fixes: https://github.com/servo/servo/issues/36764
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Signed-off-by: Simon Sapin <simon@igalia.com>
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