Allow breaking subtree for element-iterator

[Tidyzq]

Added [Symbol.iterator] for ElementIterator, which can support subtree-level breaking:

• Passing false into the iterator (aka iterator.next(false)) will skip the following subtree
• Breaking the iterator will skip the whole iteration

Examples:

    for (let node of glassEasel.ElementIterator.create(
      g,
      glassEasel.ElementIteratorType.ShadowDescendantsRootFirst,
      Object,
    )) {
        // iterate here
    }