The Build Action of the `tsconfig.json` file has to be set to `Content`. 
Otherwise Visual Studio ignores this file since VS 2019.

There is a downside though: After this change VS 2017 will complain about the Build Action:

> Error
> NETSDK1022
>
> Duplicate 'Content' items were included. The .NET SDK includes
> 'Content' items from your project directory by default. You can either
> remove these items from your project file, or set the
> 'EnableDefaultContentItems' property to 'false' if you want to
> explicitly include them in your project file. For more information,
> see https://aka.ms/sdkimplicititems. The duplicate items were:
> 'tsconfig.json'