fix(css): use single postcss instance

[sapphi-red]

Description

compileCSS function calls PostCSS inside and #8400 used compileCSS function inside the post-css-import's load option.
This causes a different PostCSS instance to be used inside @import and broke #13732. Also I guess running multiple PostCSS instances will hurt performance if tailwind is used since tailwind will crawl the files for every PostCSS instance.

This PR extracts the preprocessor part of compileCSS function to compileCSSPreprocessor function and uses that function in the post-css-import's load option. I found that #8400 didn't implement sourcemap support but I didn't implemented that because it would be complicated.

close #13732
refs #8400

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
analogjs	✅ success
astro	✅ success
histoire	❌ failure
iles	❌ failure
ladle	✅ success
laravel	✅ success
marko	✅ success
nuxt	✅ success
nx	✅ success
previewjs	✅ success
qwik	✅ success
rakkas	✅ success
sveltekit	✅ success
unocss	❌ failure
vite-plugin-pwa	✅ success
vite-plugin-ssr	✅ success
vite-plugin-react	✅ success
vite-plugin-react-pages	✅ success
vite-plugin-react-swc	✅ success
vite-plugin-svelte	✅ success
vite-plugin-vue	✅ success
vite-setup-catalogue	✅ success
vitepress	✅ success
vitest	✅ success

[bluwy]

Interesting, I overlooked of the case for normal CSS imports, where before #8400 it was already working without a separate compile. Looks like we indeed only need to handle preprocessing here.