Add starts_open setting to terminal panel (#54373)

## Summary

- Add `terminal.starts_open` setting so the terminal panel can open
automatically in new workspaces, like project and git panels already can
- Expose the setting through terminal settings docs, default settings,
Settings Editor metadata, and the terminal panel implementation
- I also added a matching settings page item for the existing
`git_panel.start_open` setting

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behaviour
- I decided not to add tests as the behaviour seems covered by the
existing settings tests, and similar areas don't seem to have their own
explicit tests.
- [x] Performance impact has been considered and is acceptable

Related to #51542 (issue mentions possibly adding for all panels, closed
with implementation for `git_panel`)

Release Notes:

- Added `terminal.starts_open` to control whether the terminal panel
opens automatically in new workspaces