Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

test/Spectre.Console.Tests/Utilities/Extensions/ShouldlyExtensions.cs

@@ -1,20 +1,16 @@
-using System;
-using System.Diagnostics;
+namespace Spectre.Console;
 
-namespace Spectre.Console
+public static class ShouldlyExtensions
 {
-    public static class ShouldlyExtensions
+    [DebuggerStepThrough]
+    public static T And<T>(this T item, Action<T> action)
     {
-        [DebuggerStepThrough]
-        public static T And<T>(this T item, Action<T> action)
+        if (action == null)
         {
-            if (action == null)
-            {
-                throw new ArgumentNullException(nameof(action));
-            }
-
-            action(item);
-            return item;
+            throw new ArgumentNullException(nameof(action));
         }
+
+        action(item);
+        return item;
     }
-}
+}