`PageTracker` tracks output to determine whether it‘s likely to take up a whole page. This doesn‘t need to be super precise, but we can use it for `SimplePager` and to avoid invoking the system pager unnecessarily.

One simplifying assumption is that we don‘t need `page?` to return `true` on the basis of an incomplete line. Long lines should be counted as multiple lines, but we don‘t have to transition from `false` to `true` until we see a newline.