fix(gatsby-plugin-sass): Add support for additionalData option

[laneparton]

Description

Adds additionalData option to gatsby-plugin-sass

Documentation

https://webpack.js.org/loaders/sass-loader/#additionaldata

Related Issues

Fixes #36085

[laneparton]

Still WIP - I'm happy to add to the README

[LekoArts]

The unit tests are currently failing () because the snapshots need an update.

And I agree, the README could use an update. A new heading called additionalData under https://github.com/laneparton/gatsby/tree/gatsby-plugin-sass-additionalData/packages/gatsby-plugin-sass#other-options should do it 👍

[laneparton]

Let me know how I can make any improvements :)

[LekoArts]

README looks good to me, unit tests still need an update though. Do you need help with that? :)

[laneparton]

README looks good to me, unit tests still need an update though. Do you need help with that? :)

I'm a little fresh on writing tests, but I gave it a shot :) Let me know if you have any feedback!

One thing I'm not 100% on, was the Joi schema. I updated it to reflect the types listed here: https://webpack.js.org/loaders/sass-loader/#additionaldata - but I wasn't sure what part Joi.uknown(true) plays in the plugin schema - I had just copied it from the implementation definition.

[LekoArts]

The unknown relates to this: https://joi.dev/api/?v=17.6.0#objectunknownallow

But since this isn't an object that's not needed

[LekoArts]

👍