Add configuration files to help with repo upkeep

[dominikag2]

• nuget.config - makes sure packages are retrieved from the correct sources
• global.json - ensures the right version of the .NET SDK (specific or within a range) is used for the solution
• Directory.Packages.props - defines the package versions to use solution-wide, which keeps versions consistent and easier to manage. When using this file, elements in .csproj files only have the package name, not the version.
• Directory.Build.props - defines MSBuild configuration for the entire solution, like .NET version, C# nullable compilation settings, etc.
• .editorconfig - defines C# code styles and syntax guidance. VS Code, Rider, and VS will detect this file and provide recommendations or errors for C# usage.