Scripts: Fix CSS imports not minified

[benoitchantre]

Runs PostCSS on the imported files.

Closes #55885.

What?

Allows CSS imports to be minified.

Why?

The files produced by npm run build should be minified.

How?

The webpack config is adjusted to use PostCSS on imported files also.

Testing Instructions

Use custom webpack.config.js and set an entry for a stylesheet.
In the stylesheet, add an import to get the content of another stylesheet.
Run the build script.

You shoud get a dist file with minified content.
Without this fix, the content that is imported is not minified.

See #55885

Testing Instructions for Keyboard

[github-actions]

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[gziolo]

Thank you for opening PR with the fix so quickly and confirming it works.

[gziolo]

I included a changelog entry for the package in 957f570, and I’m enabling auto-merge on this PR.

[github-actions]

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