Adds `data-next-font` data attribute to the preload tag if added by
`@next/font`.
```js
// Using `size-adjust` fallback font.
<link data-next-font="size-adjust" rel="preload" href="..." as="font" type="font/woff2" crossorigin="anonymous">
// Not using `size-adjust` fallback font.
<link data-next-font="" rel="preload" href="..." as="font" type="font/woff2" crossorigin="anonymous">
```
If no fonts are preloaded, the tag is added on the preconnect tag.
Fixes NEXT-350
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
e8fcf8b569