Only allow use pnpm

[ppchart]

This PR is used to prompt when using yarn or npm or cnpm to install dependencies, only pnpm is supported

But there is a bug when using npm to install:
the execution of preinstall is after install
so when this prompt is displayed, the dependent packages have already been installed.

has another solution is:
"preinstall": "npx only-allow pnpm", it's use only-allow package be supported by pnpm
This solution has the same bug as above

---

这个 PR 用来在使用 yarn 或 npm 或 cnpm 安装依赖时，进行提示只支持 pnpm

但使用 npm 安装时有一个 bug：
preinstall 的执行是在 install 安装之后
所以当出现这个提示的时候，依赖包其实已经安装好了。（只能起到提示的作用）

还有一个方案是使用 pnpm 提供的包 (only-allow)[https://github.com/pnpm/only-allow]：

// package.json
"scripts": {
    "preinstall": "npx only-allow pnpm"
}

这个方案也会出现上面说到的 npm 的 bug

[robinv8]

Thanks for the contribution