Add Roslynator (OSOE-1260)

[Piedone]

Roslynator are additional C# code analyzers maintained by the .NET team. Let's add it.

• Add Roslynator from NuGet.
• Configure each of the analyzers in Roslynator consciously: based on our practices and existing OSOCE codebase, do we need a particular rule, and if yes, configured how? Is it duplicating any of the rules from other analyzer packages (if yes, decide on which one to use)?
• Follow https://github.com/Lombiq/.NET-Analyzers#adding-a-new-analyzer.
• Fix the issues Roslynator uncovered in OSOCE.

Jira issue