updating node-fetch to v3

https://github.com/angular/angularfire/blob/a26676c69790b9a236dbc08504db705f983bd8fc/site/src/shortcodes/includecode/fetch.js#L17-L31
to use latest node-fetch version you would need to use import instead of require
```js
async function fetchCode(githubPath) {
const fetch = globalThis.fetch || import('node-fetch').then(mod => mod.default);
const githubApiUrl = convertToGitHubApiUrl(githubPath);
const response = await fetch(githubApiUrl);
return response.text();
}
```

打开 site/src/shortcodes/includecode/fetch.js 并检查第 17-31 行，这些行目前使用 require 引入 node-fetch。检查最新版本的 node-fetch 如何与 globalThis.fetch 一起加载，然后验证 fetchCode 仍能成功获取 GitHub API 的响应文本。

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