## Summary of the Pull Request
This adds a new action, `clearBuffer`. It accepts 3 values for the `clear` type:
* `"clear": "screen"`: Clear the terminal viewport content. Leaves the scrollback untouched. Moves the cursor row to the top of the viewport (unmodified).
* `"clear": "scrollback"`: Clear the scrollback. Leaves the viewport untouched.
* `"clear": "all"`: (**default**) Clear the scrollback and the visible viewport. Moves the cursor row to the top of the viewport (unmodified).
"Clear Buffer" has also been added to `defaults.json`.
## References
* From microsoft/vscode#75141 originally
## PR Checklist
* [x] Closes#1193
* [x] Closes#1882
* [x] I work here
* [x] Tests added/passed
* [ ] Requires documentation to be updated
## Detailed Description of the Pull Request / Additional comments
This is a bit tricky, because we need to plumb it all the way through conpty to clear the buffer. If we don't, then conpty will immediately just redraw the screen. So this sends a signal to the attached conpty, and then waits for conpty to draw the updated, cleared, screen back to us.
## Validation Steps Performed
* works for each of the three clear types as expected
* tests pass.
* works even with `ping -t 8.8.8.8` as you'd hope.
6268a4779c
