feat(index): add ability to read tree and write tree

lib/src/tree.dart

@@ -0,0 +1,26 @@
+import 'dart:ffi';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/tree.dart' as bindings;
+import 'util.dart';
+
+class Tree {
+  /// Initializes a new instance of [Tree] class from provided
+  /// pointers to repository object and oid object in memory.
+  ///
+  /// Should be freed with `free()` to release allocated memory.
+  Tree(Pointer<git_repository> repo, Pointer<git_oid> id) {
+    libgit2.git_libgit2_init();
+    _treePointer = bindings.lookup(repo, id);
+  }
+
+  late final Pointer<git_tree> _treePointer;
+
+  /// Pointer to memory address for allocated tree object.
+  Pointer<git_tree> get pointer => _treePointer;
+
+  /// Releases memory allocated for tree object.
+  void free() {
+    bindings.free(_treePointer);
+    libgit2.git_libgit2_shutdown();
+  }
+}