Introduce Typescript build process, generate types

Now that there is a manually written `index.js` as the entrypoint, we're duplicating a lot of work between `index.js` and `index.d.ts` that is error prone. Write an `index.ts` that replaces both in the repo, and a build process that produces both of the original files for NPM packaging.

从现有的 index.js 和 index.d.ts 入口文件开始，然后确定如何让 index.ts 替换它们，同时保持 package 接口不变。围绕为 NPM 打包生成这两个原始文件来定义构建流程；当生成的输出与 package 所需的入口点一致时，即表示完成。

由索引模型根据 Issue 内容生成。