Conpty can send a final screen update after ClosePseudoConsole which waits for the socket
to be drained before continuing. When both the socket draining and closing occurs on the
same thread a deadlock can occur which locks up VS Code's UI, the only ways to fix are
to close the client or kill the misbehaving conhost instance. This change moves the
handling of the conout named pipe to a Worker which bumps the minimum Node version
requirement to 12 (10 with a flag). This change may also have positive performance
benefits as called out in microsoft/vscode#74620

The worker change also happens for winpty, it seems to work there too.

Fixes #375