fix(postcss): too many open files error

[sonofmagic]

Hello, Unocss team, thank you for your work.

I encountered a "too many open files" error when using @unocss/postcss. This issue often occurs on Windows systems when too many files are opened simultaneously, leading to build failures.

I noticed that tailwindcss also had this problem and has already been fixed.

So I submitted a PR to address this issue in the same way as tailwindcss.

References

Async PostCSS Plugin on Windows 10 with too many parsed files (~10k) yields a webpack error (EMFILE: too many open files)

Batch reading content files to prevent too many open files error

https://github.com/tailwindlabs/tailwindcss/blob/master/src/lib/expandTailwindAtRules.js#L142

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