Add cssnano to postcss-primer-preset package and move custom properties fallback

[jonrohan]

This updates the postcss-primer-preset package to use cssnano to minify the CSS output.

• Include cssnano in the postcss-primer-preset package.
• Move the custom properties fallback out of the react package rollup config and into the preset

This shouldn't have any effect on components other than a minified output for the CSS.

[changeset-bot]

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Latest commit: bf9281b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	96.54 KB (0%)
packages/react/dist/browser.umd.js	96.82 KB (0%)