Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

src/Spectre.Console.Analyzer/Descriptors.cs

@@ -1,5 +1,3 @@
-using System.Collections.Concurrent;
-using Microsoft.CodeAnalysis;
 using static Microsoft.CodeAnalysis.DiagnosticSeverity;
 using static Spectre.Console.Analyzer.Descriptors.Category;
 
@@ -75,4 +73,4 @@ public static class Descriptors
             Usage,
             Warning,
             "Avoid prompting for input while a current renderable is running.");
-}
+}