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.Testing/Fakes/FakeCapabilities.cs

@@ -0,0 +1,17 @@
+namespace Spectre.Console.Testing
+{
+    public sealed class FakeCapabilities : IReadOnlyCapabilities
+    {
+        public bool Ansi { get; set; }
+
+        public bool Links { get; set; }
+
+        public bool Legacy { get; set; }
+
+        public bool Tty { get; set; }
+
+        public bool Interactive { get; set; }
+
+        public bool Unicode { get; set; }
+    }
+}