Add example infrastructure

* Add "Shared" projects for all examples
* Update "Colors" example with better TrueColor demo
* Use same namespace for all examples

examples/Cli/Injection/IGreeter.cs

@@ -1,6 +1,4 @@
-using System;
-
-namespace Injection
+namespace Spectre.Console.Examples
 {
     public interface IGreeter
     {
@@ -11,7 +9,7 @@ namespace Injection
     {
         public void Greet(string name)
         {
-            Console.WriteLine($"Hello {name}!");
+            AnsiConsole.WriteLine($"Hello {name}!");
         }
     }
 }