fix: emit css source map in bundleless mode

[coderbaozi]

Summary

Related Links

close: #595

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[CLAassistant]

All committers have signed the CLA.

[SoonIter]

could you please add a test case in tests/integration/sourcemap?

it would be

bundle: false,
output: {
	soucemap: {
		css: true
	}
}

thanks

[Timeless0911]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

@coderbaozi Hi, thanks for your contribution. Can you sign the CLA to let CI pass?

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[SoonIter]

thanks for your contribution ❤️