Double/half the width/height of the window

[isaacy2012]

I use multiple monitors of equal size, and sometimes I want a window to span both displays.
I often have the window on the left side and want to make it span both screens, and then return back to before.

This has been discussed before but not implemented: #984, but I think a doubling/halving implementation is much easier than the complexity of ascertaining which monitors to combine.

Happy to implement and make a PR for this feature.

[rxhanson]

Thanks for looking to contribute! This makes sense to me, and I'm good with merging a good PR for it.

My preferred way of handling additional window action contributions is that they are not added to the UI at first. There are a number of examples of this referenced in the terminal commands readme: https://github.com/rxhanson/Rectangle/blob/main/TerminalCommands.md#add-an-extra-centering-command-with-custom-size

Let me know if you need anything else to get started.