## Summary of the Pull Request
targets #20010
This adds support for the `OSC 777 ; notify ; title ; body ST` sequence.
This allows client applications to send a notification to the Terminal.
When this notification is clicked, it summons the terminal window that
sent it.
## Validation Steps Performed
```pwsh
# in PowerShell. Terminal should not be focused.
sleep 2; Write-Output "`e]777;notify;Hello;This is a notification`a"
```
## PR Checklist
- [X] Closes#7718
- [ ] Documentation updated
- [X] Schema updated (if necessary)
Heavily based on #19938
Co-authored by @zadjii-msft
93bdbfaa3d