Added initial support for rendering composites

This is far from complete, but it's a start
and it will enable us to create things like tables
and other complex objects in the long run.

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

@@ -0,0 +1,18 @@
+using System;
+
+namespace Spectre.Console.Internal
+{
+    internal sealed class MarkupToken
+    {
+        public MarkupTokenKind Kind { get; }
+        public string Value { get; }
+        public int Position { get; set; }
+
+        public MarkupToken(MarkupTokenKind kind, string value, int position)
+        {
+            Kind = kind;
+            Value = value ?? throw new ArgumentNullException(nameof(value));
+            Position = position;
+        }
+    }
+}