This PR adds support for prepending sass code before the actual entry file.
It's common for developers to import their sass mixins and variables once on their project config so they don't need to import them on every file that requires it. Frameworks like gatsby and nuxt.js already support that handy feature.
The way it works is:
```
/// next.config.js
module.exports = {
experimental: {
sassOptions: {
prependData: `
/// Scss code that you want to be
/// prepended to every single scss file.
`,
},
},
}
```
Fixes#11617 and duplicates
e66bcfa838