perf(css): improve postcss config resolve

[bluwy]

Description

With many parallel CSS file requests (especially with preTransformRequest), each CSS will try to resolve the postcss config because the cache has not been filled. This is slow as we only need one CSS file to resolve it, while others can wait for that one result. Otherwise all CSS files will resolve their own postcss config at once, causing slowness.

On top of that, postcss-load-config takes strangely long to load (2s for me), especially if you don't have a config at all. So we need to reduce calling it.

This PR does the above by caching the promise of the config resolve, instead of it's result.

Additional context

Debugging the slowness in https://github.com/sveltejs/vite-plugin-svelte/tree/main/packages/playground/big

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Dialect option can be removed as it was only separated because sugarss mutates the config to add the sugarss parser. Down the code I've refactored to not mutate the config so we can keep a single cache as before.

[patak-dev]

This is awesome 🔥