Add the ability to quit all terminal instances. Doing this separately from the window layout saving ones to lessen the number of 1k+ line monsters I make y'all review.
## References
#11083
## PR Checklist
* [x] Closes#11081
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [x] Schema updated.
## Detailed Description of the Pull Request / Additional comments
- Warn the user before they do so to give a chance to cancel
- Percolate a QuitAll event up to the monarch who then directs each peasant to clsoe.
- Leave a window-layout-saving-sized hole to add that feature on top
## Validation Steps Performed
- quit with one window (from the monarch)
- quit from the monarch with multiple windows
- quit from a peasant
- cancel the quit dialog
bee6fb4368
