Plugins: provide a DiagnosticAnalyzer to ensure IPlugin implementations follow some essential requirements.

[CaffeinatedCoder]

Coral's plugin system has three basic requirements that need to be met so that plugin implementations can be loaded and executed successfully:

1. plugin assemblies must provide only a single class implementing IPlugin
2. every implementation of IPlugin needs to be a public class ...
3. ... and needs to provide a public, parameter-less constructor

One possible solution to make this possible would be providing a Roslyn analyzer, that prevents compiling code, that doesn't fulfill all of the aforemenioned prerequisites.