[w3c/webcomponents] HTMLOptionsCollection's length attribute must have CEReactions (#596)

[`HTMLOptionsCollections`](https://html.spec.whatwg.org/#htmloptionscollection)'s [`length`](https://html.spec.whatwg.org/#dom-htmloptionscollection-length) attribute must have `CEReactions` since this attribute, on setting, can remove option elements, which in turn may disconnect descendent custom elements if there are any.

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