This introduces a `CollectionSource` trait as an alternative to
`CollectionFilter` in `HTMLCollection`, allowing collections to provide
elements directly via a custom iterator rather than filtering a full
subtree traversal, which would otherwise be rather inefficient for
smaller iterable sequences of options that can be determined without
traversing the whole subtree again.
The newly implemented `selectedOptions` attribute on the `<select>`
element uses this to iterate only the element's list of options that are
marked as selected.
Testing: 14 assertions in the existing WPT went from failing to passing.
Fixes#15522.
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Signed-off-by: Jacob Adam <software@jacobadam.net>
