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/MarkupParser.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+
+namespace Spectre.Console.Internal
+{
+    internal static class MarkupParser
+    {
+        public static IMarkupNode Parse(string text)
+        {
+            using var tokenizer = new MarkupTokenizer(text);
+            var root = new MarkupBlockNode();
+
+            var stack = new Stack<MarkupBlockNode>();
+            var current = root;
+
+            while (true)
+            {
+                var token = tokenizer.GetNext();
+                if (token == null)
+                {
+                    break;
+                }
+
+                if (token.Kind == MarkupTokenKind.Text)
+                {
+                    current.Append(new MarkupTextNode(token.Value));
+                    continue;
+                }
+                else if (token.Kind == MarkupTokenKind.Open)
+                {
+                    var (style, foreground, background) = MarkupStyleParser.Parse(token.Value);
+                    var content = new MarkupBlockNode();
+                    current.Append(new MarkupStyleNode(style, foreground, background, content));
+
+                    current = content;
+                    stack.Push(current);
+
+                    continue;
+                }
+                else if (token.Kind == MarkupTokenKind.Close)
+                {
+                    if (stack.Count == 0)
+                    {
+                        throw new InvalidOperationException($"Encountered closing tag when none was expected near position {token.Position}.");
+                    }
+
+                    stack.Pop();
+
+                    if (stack.Count == 0)
+                    {
+                        current = root;
+                    }
+                    else
+                    {
+                        current = stack.Peek();
+                    }
+
+                    continue;
+                }
+
+                throw new InvalidOperationException("Encountered unkown markup token.");
+            }
+
+            if (stack.Count > 0)
+            {
+                throw new InvalidOperationException("Unbalanced markup stack. Did you forget to close a tag?");
+            }
+
+            return root;
+        }
+    }
+}