RNSlider Windows has a built authoring issue on how it consumes RNW

```
\node_modules\react-native-windows\PropertySheets\External\Microsoft.ReactNative.Uwp.CppLib.targets(12,5): warning MSB4011: "\node_modules\react-native-windows\Microsoft.ReactNative.Cxx\Microsoft.ReactNative.Cxx.vcxitems" cannot be imported again. It was already imported at "\node_modules\@react-native-community\slider\windows\SliderWindows\SliderWindows.vcxproj (80,5)". This is most likely a build authoring error. This subsequent import will be ignored.
```

The way RNW is being consumed from SliderWindows needs to be fixed/updated. The proper way is to consume via the RNW targets and props now.

I will likely open a PR to address this soon.

Start with windows/SliderWindows/SliderWindows.vcxproj and the RNW PropertySheets/External/Microsoft.ReactNative.Uwp.CppLib.targets path shown in the warning. Review how RNW is currently consumed there and compare the targets and props approach described in the issue. Done means the project uses that approach and the duplicate-import build warning no longer appears.

Written by the indexing model from the issue text.