Rename %IteratorPrototype%.index to lastIndex? about proposal-intl-segmenter

[littledan]

Do you think index is unclear? I am not sure if we need to be consistent with RegExp.

[gibson042]

My default preference is to match existing terms and semantics where possible, unless there is good reason not to. index was better than position, and %IteratorPrototype%.lastIndex might be better than %IteratorPrototype%.index but I'm not sure.

Thinking about it now, however, I like the symmetry of supporting same-shape destructuring between iterators and iterator results (e.g., { index } = boundaries.next() and boundaries.following(); { index } = boundaries) better than the symmetry of matching RegExp instances.