Scripts: Import CSS files before optimization

[benoitchantre]

What?

Import CSS files before the minification process.

Closes #61112.

Why?

Small performance improvement for people using CSS imports and the @wordpress/scripts package.

How?

postcss-import is run before cssnano

Testing Instructions

1. Scaffold a block
2. Rename your main stylesheet from .scss to .css
3. Create 2 CSS files
4. Import these files in the main stylesheet
5. Run npm run build
6. See that you have a resulting stylesheet that is minified (only one line instead of two)

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[benoitchantre]

@gziolo A few tests have failed at npm install. Should I rebase and force push this PR?

Would you have some time to review it?

[gziolo]

It works as advertised in my testing. It will need a rebase to resolve conflicts with the latest changes in trunk.

It would be also nice to add an entry in the changelog file under ## Unreleased section:

### Enhancements

- Inlines CSS files imported from other CSS files before optimization in the `build` command ([#61121](https://github.com/WordPress/gutenberg/pull/61121)).

[gziolo]

Thank you so much for taking it to the finish line.