fix: not push non-async css files into map

[clarkdo]

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• Bugfix
• Feature
• Code style update
• Refactor
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Other information:

Fix nuxt/nuxt#5190

Since #7902 has already implemented initial and async for css, so make fileMapper only include async css files same as js files for avoiding duplicate style tags.

[yyx990803]

Can you provide some more context for this fix?

[clarkdo]

@yyx990803 It happened in extracting css.
When I extracted css from entrypoint, the css file will be in initial in manifest , then during, renderring, it will also be put into async which is used in renderStyles since all initial files except js are all put into async.

[AlexDubok]

Hi! Found this pr because I have the same issue. Initial css files are being put into async files array, wich is then combined here with initial files array and used to render style links.