[feat]: add gatsby-plugin-cxs

[DSchau]

Summary

CXS is a great library for CSS in JS w/ React. Let's create a plugin so that others can get some SSR benefits and CSS hydration with Gatsby.

Motivation

• We have a number of other plugins which implement similar functionality, e.g. gatsby-plugin-emotion, gatsby-plugin-styled-components, etc.
• We don't want to prohibit anyone from using Gatsby because their preferred styling solution isn't supported, and Gatsby is extremely pluggable to enable this :)

General Approach

• Create packages/gatsby-plugin-cxs (see previously mentioned CSS in JS libraries for more info/boilerplate)
• Implement the SSR API from Aphrodite
• PR and experience the ❤️ from all of those who use and like Cxs

[ctxhou]

Is this issue taken? If not, I would like to work on this 😄

[DSchau]

@ctxhou we'd love for you to take it if you're able! Looking forward to the PR, and let any of us @gatsbyjs/core know if we can help!

[tbaustin]

@ctxhou have you completed this or do you need help?

[ctxhou]

Hi @tbaustin, I am working on this!
I will send the pull request later 😄

[amberleyromo]

Hi @ctxhou! Still working on this?

[ctxhou]

hey @amberleyromo, I've send the PR and wait for review 😄

[DSchau]

This was done in #8878. Thanks @ctxhou!