Allow escaping quotes in string

Upsilon/Parser/Lexer.cs

@@ -144,6 +144,11 @@ namespace Upsilon.Parser
             while (_position < _text.Length)
             {
                 _position++;
+                if (Current == '\\' && Next == '"')
+                {
+                    sb.Append("\"");
+                    _position += 2;
+                }
                 if (Current == '"')
                     break;
                 sb.Append(Current);