在 VS Code 扩展中添加自动完成功能需要使用 CompletionItemProvider 接口。在保留建议的情况下，可以使用以下代码实现：

import {
  CompletionItem,
  CompletionItemKind,
  CompletionItemProvider,
  TextDocument,
  Position,
  CancellationToken,
} from 'vscode';

class MyCompletionItemProvider implements CompletionItemProvider {
  public async provideCompletionItems(
    document: TextDocument,
    position: Position,
    token: CancellationToken,
  ): Promise {
    // 获取建议列表
    const suggestions = await this.getSuggestions(position);

    // 转换建议为 CompletionItem 对象
    const completionItems = suggestions.map(suggestion => {
      const item = new CompletionItem(suggestion.label);
      item.kind = CompletionItemKind[suggestion.kind];

      return item;
    });

    return completionItems;
  }

  private async getSuggestions(position: Position): Promise {
    // 从 API 获取建议列表
    // ...

    return suggestions;
  }
}

interface SuggestionItem {
  label: string;
  kind: string;
}

// 在扩展入口中注册 CompletionItemProvider
context.subscriptions.push(
  languages.registerCompletionItemProvider(
    'myLanguage', // 改为您的语言标识符
    new MyCompletionItemProvider(),
  ),
);