feat(treebuilder): add bindings and api

lib/src/treebuilder.dart

@@ -0,0 +1,77 @@
+import 'dart:ffi';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/treebuilder.dart' as bindings;
+import 'repository.dart';
+import 'oid.dart';
+import 'enums.dart';
+import 'tree.dart';
+import 'util.dart';
+
+class TreeBuilder {
+  /// Initializes a new instance of [TreeBuilder] class from provided
+  /// [Repository] and optional [Tree] objects.
+  ///
+  /// Should be freed with `free()` to release allocated memory.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  TreeBuilder(Repository repo, [Tree? tree]) {
+    _treeBuilderPointer = bindings.create(
+      repo.pointer,
+      tree?.pointer ?? nullptr,
+    );
+  }
+
+  late final Pointer<git_treebuilder> _treeBuilderPointer;
+
+  /// Pointer to memory address for allocated tree builder object.
+  Pointer<git_treebuilder> get pointer => _treeBuilderPointer;
+
+  /// Returns the number of entries listed in a tree builder.
+  int get length => bindings.entryCount(_treeBuilderPointer);
+
+  /// Writes the contents of the tree builder as a tree object.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Oid write() => Oid(bindings.write(_treeBuilderPointer));
+
+  /// Clears all the entires in the tree builder.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void clear() => bindings.clear(_treeBuilderPointer);
+
+  /// Returns an entry from the tree builder from its filename.
+  ///
+  /// The returned entry is owned by the tree builder and should not be freed manually.
+  ///
+  /// Throws [ArgumentError] if nothing found for provided filename.
+  TreeEntry operator [](String filename) {
+    return TreeEntry(bindings.getByFilename(_treeBuilderPointer, filename));
+  }
+
+  /// Adds or updates an entry to the tree builder with the given attributes.
+  ///
+  /// If an entry named filename already exists, its attributes will be updated with
+  /// the given ones.
+  ///
+  /// By default the entry that you are inserting will be checked for validity;
+  /// that it exists in the object database and is of the correct type.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void add(String filename, Oid id, GitFilemode filemode) {
+    bindings.add(
+      _treeBuilderPointer,
+      filename,
+      id.pointer,
+      gitFilemodeToInt(filemode),
+    );
+  }
+
+  /// Removes an entry from the tree builder by its filename.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void remove(String filename) =>
+      bindings.remove(_treeBuilderPointer, filename);
+
+  /// Releases memory allocated for tree builder object.
+  void free() => bindings.free(_treeBuilderPointer);
+}