Finally implements the `newWindow` action. It does so by
`ShellExecute`ing `wt.exe` with commandline args corresponding to the
ones that would create the same `NewTerminalArgs`. This works with #8898
and #9118 to allow new windows (even with `windowingBehavior:
useExisting`)
This is taken from my auto-elevate branch, hence the references to
elevation
References #5000
References projects/5
References #8898
References #9118Closes#1051