Ability to override target framework in ElectronDevMode

Electron.NET is targeting multiple frameworks, including deprecated ones.

While using ElectronDevMode we might use it as a submodule of an existing product where only some targets are available.

In order to enhance developer experience, add the ability to overload the target framework from the `Directory.Build.props` barrier.

In addition, this will solve the Visual Studio message '`This project is targeting a version of .NET that is not installed`' for solutions without multiple target frameworks.