chore(doc):Add babel preset css prop to jest-preprocess.js

[KonnorRogers]

Description

Add @emotion/babel-preset-css-prop to the jest-preprocess.js file. This allows for the stringificaiton of CSS properties in snapshot testing. Currently, with Emotion 10, if the babel option is not set, you will not have readable css properties. @emotion/babel-preset-css-prop is shipped by default with Emotion 10 and does not need to be installed seperately by npm.

Documentation

https://github.com/emotion-js/emotion/tree/master/packages/babel-preset-css-prop

[Ekwuno]

Thanks for pointing this out. According to the official documentation, it checks out.

[gatsbot]

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