fix: add a background color to test nested css

[gabyzif]

Summary

• Integrated PostCSS with postcss-nesting to handle CSS nesting in the Vite build.
• Updated vite.config.js to support ESM imports for PostCSS plugins.
• Ensured proper CSS processing and compatibility with legacy projects.

Testing Plan

This PR ENV has an example of it working (using a background color as an example)

Before adding nested css, when building Nancy localy for Prod the console will show this warning:

after (no warning):

• Was this tested locally? If not, explain why.
• {explain how this has been tested, and what, if any, additional testing should be done}

Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/REPLACEME

[ilyusha]

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[github-actions]

PR Preview Action v1.4.8
🚀 Deployed preview to https://mParticle.github.io/aquarium/pr-preview/pr-428/
on branch gh-pages at 2024-09-30 13:56 UTC