Adds support for the `windowingBehavior` global setting. This setting
controls how mutiple instances of `wt` behave in the absence of the `-w`
parameter. This setting has three values:
* `"useNew"`: (default) Multiple `wt` invocations (without the `-w`
param) always create new windows.
* `"useAnyExisting"`: When starting a new `wt`, we'll instead default to
any existing windows. `wt -w -1` will still create new windows.
* `"useExisting"`: Similar to `useAnyExisting`, but limits to
windows on the current desktop.
The IVirtualDesktopManager interface is _very_ limited. Hence why we
have to track the HWNDs manually, and ask if they're on the current
desktop.
## Validation Steps Performed
I've been playing with it for a week now.
References #5000
References projects/5
References #8898
Spec'd in #8135Closes#2227
Closes https://github.com/microsoft/terminal/projects/5#card-51431448
Closes https://github.com/microsoft/terminal/projects/5#card-51431433
69318d3ba1