Allows users to import all.scss in their own sass files using:

`@import "govuk-frontend"`

May only be supported by Webpack at the moment
https://webpack.js.org/guides/package-exports/#conditions

Allows users to directly import certain sass files, e.g:

```
@import "govuk-frontend/govuk/base";
@import "govuk-frontend/govuk/all.scss";
@import "govuk-frontend/govuk/all";
@import "govuk-frontend/govuk/core/all";
@import "govuk-frontend/govuk/objects/all";
```

Similar to adding "./govuk/" to package.json, this lets tools know that these
paths are also available.