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/Extensions/StringExtensions.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Text;
+
+namespace Spectre.Console.Internal
+{
+    internal static class StringExtensions
+    {
+        // Cache whether or not internally normalized line endings
+        // already are normalized. No reason to do yet another replace if it is.
+        private static readonly bool _alreadyNormalized
+            = Environment.NewLine.Equals("\n", StringComparison.OrdinalIgnoreCase);
+
+        public static int CellLength(this string text, Encoding encoding)
+        {
+            return Cell.GetCellLength(encoding, text);
+        }
+
+        public static string NormalizeLineEndings(this string text, bool native = false)
+        {
+            var normalized = text?.Replace("\r\n", "\n")
+                ?.Replace("\r", string.Empty);
+
+            if (native && !_alreadyNormalized)
+            {
+                normalized = normalized.Replace("\n", Environment.NewLine);
+            }
+
+            return normalized;
+        }
+
+        public static string[] SplitLines(this string text)
+        {
+            return text.NormalizeLineEndings().Split(new[] { '\n' }, StringSplitOptions.None);
+        }
+    }
+}