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();
}
```

Open site/src/shortcodes/includecode/fetch.js and inspect lines 17-31, which currently use node-fetch with require. Check how the latest node-fetch version is loaded alongside globalThis.fetch, then verify that fetchCode still retrieves the GitHub API response text successfully.

Written by the indexing model from the issue text.