Additional Set method support: Pending support in FireFox, Opera, and Node

[NullVoxPopuli]

Pending node support, because that's where the tests run.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/difference
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/intersection
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/isDisjointFrom
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/isSubsetOf
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/isSupersetOf
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/symmetricDifference
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/union