[megathread] Chromium parity for tab drag/drop

[HotCakeX]

maintainer edit

This is blocked on a TabView v3 from WinUI.

### Tasks
* [ ] Tabs should make a new window as soon as their torn out
* [ ] Torn-out tabs need to be able to snap to snap positions (maximized, snap layouts) without dropping first
* [ ] Multiple tabs can't be dragged/dropped all at once
* [ ] Dragging a tab in a window with a single tab should just drag the window

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Uploaded a video to show exactly what I'm talking about. In the video below I'm comparing tab tear out and joining with Microsoft Edge. Windows Terminal's tab experience should be like Edge because it's currently hard to work with it when a tab can't join another window unless that window is visible.

https://github.com/microsoft/terminal/assets/118815227/195417e9-0034-400b-bcf4-813d8f264563

[zadjii-msft]

You know what, I'm gonna promote this to be a little more broad than just "merge when the window is obscured". Instead, I'm just gonna use this as the "chromium tab drag/drop parity" thread. I've got other notes in #14900 on this topic too, which I'm gonna merge into the OP.

The current WinUI TabView implementation leaves a lot to be desired. We're working with them on getting more advanced APIs for a better UX here.