Surrounding pairs v2

[pokey]

Once we have #484, we can greatly simplify our surrounding pairs code. In the process, we'd like to do the following:

• [ ] Only deal with expanding from a single position, so we can remove the backwards pass to extend past selection start
• [ ] Refactor text-based item scope stage to leverage surrounding pairs code. We can probably use a greatly simplified version of the algorithm suggested in #357, because that one tries to handle ranges, not just positions
• [ ] Support "every" for surrounding pairs