Implement NbtIO and some unit tests using the HelloWorld NBT

lib/nbt/InvalidNBTDataException.dart

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+class InvalidNbtDataException implements Exception {}

lib/nbt/NbtIo.dart

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+import 'package:libac_flutter/nbt/Stream.dart';
+import 'package:libac_flutter/nbt/Tag.dart';
+import 'package:libac_flutter/nbt/impl/CompoundTag.dart';
+
+class NbtIo {
+  static ByteLayer _io = ByteLayer();
+
+  // Handle various helper functions here!
+
+  static Future<void> _read(String file) async {
+    _io = ByteLayer();
+
+    await _io.readFromFile(file);
+  }
+
+  // This function will read the file and check if it is infact gzipped
+  static Future<CompoundTag> read(String file) async {
+    await _read(file);
+    if (_io.readByte() == TagType.Compound.byte) {
+      _io.resetPosition();
+      return Tag.readNamedTag(_io) as CompoundTag;
+    } else {
+      // Is likely gzip compressed
+      return readCompressed(file);
+    }
+  }
+
+  static Future<CompoundTag> readCompressed(String file) async {
+    _io = ByteLayer();
+    await _io.readFromFile(file);
+    await _io.decompress();
+    _io.resetPosition();
+    return Tag.readNamedTag(_io) as CompoundTag;
+  }
+
+  static Future<void> write(String file) async {
+    await _io.writeToFile(file);
+  }
+
+  static Future<void> writeCompressed(String file) async {
+    await _io.compress();
+    await _io.writeToFile(file);
+  }
+
+  static ByteLayer getStream() {
+    return _io;
+  }
+}