[cssom-1] Add missing extract class on dashed_attribute example

[saschanaz]

https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_dashed_attribute

partial interface CSSStyleDeclaration {
  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _dashed_attribute;
};

This IDL snippet does not have extract class attached and thus collected on IDL index, whereas other _camel_cased_attribute and _webkit_cased_attribute examples are not collected.

[zcorpan]

Thanks!