Codex needs (and future projects are anticipated to need as well) a
concept of display-only terminals. This refactors terminals to decouple
the PTY part from the display part, so that we can render terminal
changes based on a series of events - regardless of whether they're
being driven from a PTY inside Zed or from an outside source (e.g.
`codex-acp`).

Release Notes:

- N/A