Build: deduplicate and minify embedded styles

[jsnajdr]

Improves how we deal with style imports that we inline into the transpiled JS code. Two improvements:

• generate custom JS code for inserting the style element. It checks if the style element with the given data-wp-hash attribute already exists, and prevents inserting duplicate styles. That can reduce style duplication a lot when libraries like @wordpress/ui are bundled many times with various chunks (blocks, routes etc.)
• minify the embedded code with cssnano. Makes the transpiled JS files smaller.

We're still using postcss here for both CSS modules and minification. In the near future I'd like to switch to lightningcss but they need to release a bugfixed version first.

How to test

look at built files for:

• individual packages, like packages/ui/build-module/form/primitives/fieldset/legend.mjs
• WP script, like build/scripts/components/index.js

and verify that they both contain the new CSS-inserting code with minified CSS literals.

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Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>

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