Change IAnsiConsole to render IRenderable

This makes it possible for encoders to output better representation
of the actual objects instead of working with chopped up segments.

* IAnsiConsole.Write now takes an IRenderable instead of segments
* Calculating cell width does no longer require a render context
* Removed RenderContext.LegacyConsole
* Removed RenderContext.Encoding
* Added Capabilities.Unicode

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Markup.cs

@@ -38,7 +38,7 @@ namespace Spectre.Console
         /// <param name="value">The value to write.</param>
         public static void Markup(this IAnsiConsole console, string value)
         {
-            console.Render(MarkupParser.Parse(value));
+            console.Write(MarkupParser.Parse(value));
         }
 
         /// <summary>