feat(treebuilder): add bindings and api

test/treebuilder_test.dart

@@ -0,0 +1,85 @@
+import 'dart:io';
+
+import 'package:test/test.dart';
+import 'package:libgit2dart/libgit2dart.dart';
+import 'package:libgit2dart/src/treebuilder.dart';
+import 'helpers/util.dart';
+
+void main() {
+  late Repository repo;
+  late Tree tree;
+  final tmpDir = '${Directory.systemTemp.path}/treebuilder_testrepo/';
+  const treeSHA = 'a8ae3dd59e6e1802c6f78e05e301bfd57c9f334f';
+
+  setUp(() async {
+    if (await Directory(tmpDir).exists()) {
+      await Directory(tmpDir).delete(recursive: true);
+    }
+    await copyRepo(
+      from: Directory('test/assets/testrepo/'),
+      to: await Directory(tmpDir).create(),
+    );
+    repo = Repository.open(tmpDir);
+    tree = Tree.lookup(repo, Oid.fromSHA(repo, treeSHA));
+  });
+
+  tearDown(() async {
+    tree.free();
+    repo.free();
+    await Directory(tmpDir).delete(recursive: true);
+  });
+
+  group('TreeBuilder', () {
+    test('successfully initializes tree builder when no tree is provided', () {
+      final builder = TreeBuilder(repo);
+      expect(builder, isA<TreeBuilder>());
+      builder.free();
+    });
+
+    test('successfully initializes tree builder with provided tree', () {
+      final builder = TreeBuilder(repo, tree);
+      final oid = builder.write();
+
+      expect(builder, isA<TreeBuilder>());
+      expect(builder.length, tree.length);
+      expect(oid, tree.id);
+
+      builder.free();
+    });
+
+    test('clears all the entries in the builder', () {
+      final builder = TreeBuilder(repo, tree);
+
+      expect(builder.length, 4);
+      builder.clear();
+      expect(builder.length, 0);
+
+      builder.free();
+    });
+
+    test('successfully builds the tree builder from entry of tree', () {
+      final builder = TreeBuilder(repo);
+      final entry = tree.entries[0];
+
+      expect(() => builder[entry.name], throwsA(isA<ArgumentError>()));
+
+      builder.add(entry.name, entry.id, entry.filemode);
+      expect(builder[entry.name].name, entry.name);
+
+      builder.free();
+      entry.free();
+    });
+
+    test('successfully removes an entry', () {
+      final builder = TreeBuilder(repo, tree);
+
+      expect(builder.length, tree.length);
+
+      builder.remove('.gitignore');
+      expect(() => builder['.gitignore'], throwsA(isA<ArgumentError>()));
+      expect(builder.length, tree.length - 1);
+
+      builder.free();
+    });
+  });
+}