terminal: Drain task output on completion (#23085)

Now we ensure that task output is fully drained and printed to Zed
terminal pane on task completion.

This change depends on a recent change to alacritty_terminal crate:
https://github.com/alacritty/alacritty/commit/5e78d20c709cb1ab8d44ca7a8702cc26d779227c.

Closes https://github.com/zed-industries/zed/issues/18342

Release Notes:

- Fixed missing task terminal output on Linux for short-running commands