Moved analyzer tests to its own project

Also moves tests to `./test` which makes it possible
for all test projects to share the same .editorconfig
files and similar.

src/Spectre.Console/Internal/Backends/Ansi/AnsiConsoleBackend.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Text;
 using Spectre.Console.Rendering;
 using static Spectre.Console.AnsiSequences;
 

src/Spectre.Console/Widgets/Paragraph.cs

@@ -2,7 +2,6 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Text;
 using Spectre.Console.Rendering;
 
 namespace Spectre.Console