Added stylecop to the project
This merge request adds Stylecop to the project using Unity's Roslyn analyzer feature: https://docs.unity3d.com/Manual/roslyn-analyzers.html. This means that we now get an analysis which enforces us to use a consistent code style. It also flags missing documentation.
Unfortunately, there is currently a Unity bug https://issuetracker.unity3d.com/issues/ruleset-file-is-ignored-when-using-roslyn-analyzers which ignores the given ruleset file where we can define which rules Stylecop should enforce. However, once we update to Unity 2021, this bug should be resolved.