Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

src/Spectre.Console/Internal/Text/Encoding/HtmlEncoder.cs

@@ -1,8 +1,3 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-using Spectre.Console.Rendering;
-
 namespace Spectre.Console.Internal;
 
 internal sealed class HtmlEncoder : IAnsiConsoleEncoder

src/Spectre.Console/Internal/Text/Encoding/TextEncoder.cs

@@ -1,7 +1,3 @@
-using System.Collections.Generic;
-using System.Text;
-using Spectre.Console.Rendering;
-
 namespace Spectre.Console.Internal;
 
 internal sealed class TextEncoder : IAnsiConsoleEncoder

src/Spectre.Console/Internal/Text/Markup/MarkupParser.cs

@@ -1,7 +1,3 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-
 namespace Spectre.Console;
 
 internal static class MarkupParser

src/Spectre.Console/Internal/Text/Markup/MarkupToken.cs

@@ -1,5 +1,3 @@
-using System;
-
 namespace Spectre.Console;
 
 internal sealed class MarkupToken

src/Spectre.Console/Internal/Text/Markup/MarkupTokenizer.cs

@@ -1,6 +1,3 @@
-using System;
-using System.Text;
-
 namespace Spectre.Console;
 
 internal sealed class MarkupTokenizer : IDisposable

src/Spectre.Console/Internal/Text/StringBuffer.cs

@@ -1,6 +1,3 @@
-using System;
-using System.IO;
-
 namespace Spectre.Console;
 
 internal sealed class StringBuffer : IDisposable