Windows doesn't have signals, the default Kill doesn't actually kill
anything, and other forms of termination don't allow cleanup. So we
spawn preview processes in a new process group and send them a
CTRL_BREAK_EVENT to terminate.
However, we only do this for "pwsh" (PowerShell 7+) and unknown/
posix-ish shells, because cmd.exe and Windows PowerShell
("powershell.exe") don't always exit on Ctrl-Break. pwsh also needs
the -NonInteractive flag to exit on Ctrl-Break.
If the process wasn't given its own group, or if sending the console
control event fails, we fall back to the standard Kill (which likely
won't help, but doesn't hurt to try).
Fix#3134
fff7eda9d7