Re: [heycam/webidl] undefined can't be a member of a union type (#962)

If you insert a rule at the top of the union conversion that `If the union type includes the undefined type and V is undefined, then return the IDL value undefined`, then that allows unions that treat `null` and `undefined` as separate values (just need to also include a nullable type in the union). If one wants `null` and `undefined` to be treated the same, then just don't include `undefined` in the union. The algorithm then falls back to `If the union type includes a nullable type and V is null or undefined, then return the IDL value null`. It's not great for distinguishability (`null` and `undefined` are distinguishable but also not?), but I think that because of the existing conversion behaviour it's hard to reconcile both. Unless you hide that behind something like `[LegacyUndefinedToNull]`, but that seems pretty daunting.

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