Remove Monarch/Peasant & Make UI single-threaded (#18215)

As before, a minor refactor:
* I started off by removing the Monarch/Peasant with the goal of moving
  it into and deduplicating its functionality with `WindowEmperor`.
* Since I needed a replacement for the Monarch (= ensures that there's
  a single instance), I wrote single-instance code with a NT mutex
  and by yeeting data across processes with `WM_COPYDATA`.
* This resulted in severe threading issues, because it now started up
  way faster. The more I tried to solve them the deeper I had to dig,
  because you can't just put a mutex around `CascadiaSettings`.
  I then tried to seeif WinUI can run multiple windows on a single
  thread and, as it turns out, it can.
  So, I removed the multi- from the window threading.
* At this point I had dig about 1 mile deep and brought no ladder.
  So, to finish it up, I had to clean up the entire eventing system
  around `WindowEmperor`, cleaned up all the coroutines,
  and cleaned up all the callbacks.

Closes #16183
Closes #16221
Closes #16487
Closes #16532
Closes #16733
Closes #16755
Closes #17015
Closes #17360
Closes #17420
Closes #17457
Closes #17799
Closes #17976
Closes #18057
Closes #18084
Closes #18169
Closes #18176
Closes #18191

## Validation Steps Performed
* It does not crash ✅
* New/close tab ✅
* New/close window ✅
* Move tabs between windows ✅
* Split tab into new window ✅
* Persist windows on exit / restore startup ✅

doc/cascadia/profiles.schema.json

@@ -2372,11 +2372,6 @@
           "description": "When set to true, Windows Terminal will run in the background. This allows globalSummon and quakeMode actions to work even when no windows are open.",
           "type": "boolean"
         },
-        "compatibility.isolatedMode": {
-          "default": false,
-          "description": "When set to true, Terminal windows will not be able to interact with each other (including global hotkeys, tab drag/drop, running commandlines in existing windows, etc.). This is a compatibility escape hatch for users who are running into certain windowing issues.",
-          "type": "boolean"
-        },
         "compatibility.allowDECRQCRA": {
           "default": false,
           "description": "When set to true, the terminal will support the DECRQCRA (Request Checksum of Rectangular Area) escape sequence.",