Trailing state capture now is encapsulated in a struct `Capture` and all parsers access the data via `p.capture.trailing()` rather than directly from the writer. This is primarily to prep for the OSC parser to be able to capture the entire sequence (not just the trailing part) so we can setup libghostty for fallback handlers so libghostty implementers can have custom OSC behaviors. But, it has the benefit of making our OSC parser much cleaner too.