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/StringBuffer.cs

@@ -0,0 +1,50 @@
+using System;
+using System.IO;
+
+namespace Spectre.Console.Internal
+{
+    internal sealed class StringBuffer : IDisposable
+    {
+        private readonly StringReader _reader;
+        private readonly int _length;
+
+        public int Position { get; private set; }
+        public bool Eof => Position >= _length;
+
+        public StringBuffer(string text)
+        {
+            text ??= string.Empty;
+
+            _reader = new StringReader(text);
+            _length = text.Length;
+
+            Position = 0;
+        }
+
+        public void Dispose()
+        {
+            _reader.Dispose();
+        }
+
+        public char Peek()
+        {
+            if (Eof)
+            {
+                throw new InvalidOperationException("Tried to peek past the end of the text.");
+            }
+
+            return (char)_reader.Peek();
+        }
+
+        public char Read()
+        {
+            if (Eof)
+            {
+                throw new InvalidOperationException("Tried to read past the end of the text.");
+            }
+
+            Position++;
+            return (char)_reader.Read();
+        }
+    }
+}