Port NBT impl from LibAC Dart

NBT/API/StringBuilder.cs

@@ -0,0 +1,32 @@
+using System.Text;
+
+namespace LibAC.NBT.API
+{
+    public class StringBuilder
+    {
+        private string _buffer = string.Empty;
+
+        public StringBuilder()
+        {
+        }
+
+        public bool IsEmpty => string.IsNullOrEmpty(_buffer);
+        
+        public int Length => _buffer.Length;
+
+        public void Append(string value)
+        {
+            _buffer += value;
+        }
+
+        public void Clear()
+        {
+            _buffer = string.Empty;
+        }
+
+        public override string ToString()
+        {
+            return _buffer;
+        }
+    }
+}