Refactoring c# projects (#544)

Small Refactoring c# projects, e.g.
* add days
* remove StartupObject
* Fix Approval tests for NUnit
* Use Environment.NewLine in Xunit tests
* Added + fixed README.md
* Removed global.json

csharp.xUnit/README.md

@@ -0,0 +1,24 @@
+# Gilded Rose starting position in C# xUnit
+
+## Build the project
+
+Use your normal build tools to build the projects in Debug mode.
+For example, you can use the `dotnet` command line tool:
+
+``` cmd
+dotnet build GildedRose.sln -c Debug
+```
+
+## Run the Gilded Rose Command-Line program
+
+For e.g. 10 days:
+
+``` cmd
+GildedRose/bin/Debug/net8.0/GildedRose 10
+```
+
+## Run all the unit tests
+
+``` cmd
+dotnet test
+```