feat: add measureElement on virtual item, when we can't relay on node.isConnected

[piecyk]

As we can relay on node.isConnected to attach the RO or read size, like in lit integration #718 we need to have another option to do. In this PR #737 let's add back the measureElement on virtual item and keep it stale via measureElementCache that has bound in scope index.

${ref(virtualRow.measureElement}

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[kkx64]

@piecyk @tannerlinsley This is all good and well, but changing the type of VirtualItem to VirtualItem<T> in a minor release made our UI component builds randomly fail. Please be careful of these types of changes.