Enable Vintage Opacity (#11180)

## Summary of the Pull Request
![603-final](https://user-images.githubusercontent.com/18356694/132585665-afed3210-257a-4fee-9b43-4273a0f5cf69.gif)

Adds support for vintage style opacity, on Windows 11+. The API we're using for this exists since the time immemorial, but there's a bug in XAML Islands that prevents it from working right until Windows 11 (which we're working on backporting).

Replaces the `acrylicOpacity` setting with `opacity`, which is a uint between 0 and 100 (inclusive), default to 100.

`useAcrylic` now controls whether acrylic is used or not. Setting an opacity < 100 with `"useAcrylic": false` will use vintage style opacity.

Mouse wheeling adjusts opacity. Whether acrylic is used or not is dependent upon `useAcrylic`.

`opacity` will stealthily default to 50 if `useAcrylic:true` is set.

## PR Checklist
* [x] Closes #603
* [x] I work here
* [x] Tests added/passed
* [x] https://github.com/MicrosoftDocs/terminal/pull/416

## Detailed Description of the Pull Request / Additional comments

Opacity was moved to AppearanceConfig. In the future, I have a mind to allow unfocused acrylic, so that'll be important then. 

## Validation Steps Performed
_just look at it_

scratch/ScratchIslandApp/SampleApp/MySettings.h

@@ -45,7 +45,7 @@ namespace winrt::SampleApp::implementation
 
         WINRT_PROPERTY(winrt::hstring, ProfileName);
         WINRT_PROPERTY(bool, UseAcrylic, false);
-        WINRT_PROPERTY(double, TintOpacity, 0.5);
+        WINRT_PROPERTY(double, Opacity, .5);
         WINRT_PROPERTY(winrt::hstring, Padding, DEFAULT_PADDING);
         WINRT_PROPERTY(winrt::hstring, FontFace, L"Consolas");
         WINRT_PROPERTY(int32_t, FontSize, DEFAULT_FONT_SIZE);