## 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.
Fix startup race of resizing ConPTY
- Depending on what the timing and ordering is of the message coming in
from the signal thread, it may be applied to the startup structure
after the I/O thread has begun initializing the console buffer
structures but before it has signaled that it is done and the signal
thread is ready to make changes directly. This likely happens because
the end of the I/O thread setup has a weird unlock/lock jog for the
input thread and the signal thread might have been scheduled in the
middle of it.
- My resolution here is to ensure that the signal thread just keeps
storing the latest resize message until it is told that everything is
initialized. Whomever comes in to tell the signal thread this
information (under lock) will pickup and run the resize if one came in
before everything was ready. This should resolve the race.
## Validation Steps Performed
- o-sdn-o confirms this resolves their issue
Closes#10400
