Finish implementing - and testing SNBT

lib/nbt/Stream.dart

@@ -419,6 +419,9 @@ class StringReader {
   // Check if there's more to read
   bool get canRead => _position < _buffer.length;
 
+  // Get the number of chars seeked
+  int get getSeeked => _lastPostion - _position;
+
   // Read the next character
   String next() {
     if (canRead) {