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## Summary of the Pull Request
This PR turns all* instances of `Dispatcher().RunAsync` to WinRT coroutines 👌.
This was good coding fodder to fill my plane ride ✈️. Enjoy your holidays everyone!
*With the exception of three functions whose signatures cannot be changed due to inheritance and function overriding in `TermControlAutomationPeer` [`L44`](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalControl/TermControlAutomationPeer.cpp#L44), [`L58`](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalControl/TermControlAutomationPeer.cpp#L58), [`L72`](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalControl/TermControlAutomationPeer.cpp#L72).
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## References
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## PR Checklist
* [x] Closes#3919
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #3919
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## Detailed Description of the Pull Request / Additional comments
My thought pattern here was to minimally disturb the existing code where possible. So where I could, I converted existing functions into coroutine using functions (like in the [core example](https://github.com/microsoft/terminal/issues/3919#issue-536598706)). For ~the most part~ all instances, I used the format where [`this` is accessed safely within a locked scope](https://github.com/microsoft/terminal/issues/3919#issuecomment-564730620). Some function signatures were changed to take objects by value instead of reference, so the coroutines don't crash when the objects are accessed past their original lifetime. The [copy](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalApp/TerminalPage.cpp#L1132) and [paste](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalApp/TerminalPage.cpp#L1170) event handler entry points were originally set to a high priority; however, the WinRT coroutines don't appear to support a priority scheme so this priority setting was not preserved in the translation.
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## Validation Steps Performed
Compiles and runs, and for every event with a clear trigger repro, I triggered it to ensure crashes weren't introduced.
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