fix(css): don't override internal importer

[Laupetin]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.

Description

This PR fixes the fact, that user specified importers for sass via preprocessorOptions override the default importer from tsdown.
This breaks e.g. importing from node_modules.

For details and reproduction link, see the corresponding issue: #859

Linked Issues

Fixes: #859

Additional context

I fixed the issue by doing the minimal changes, keeping user values overriding all values expect for the special case handling of the importers array.

If user values overriding the other values would also be considered unintended behaviour, the object spread for the user values could be moved up even further:

  const result = await sass.compileStringAsync(data, {
    ...sassOptions,
    url: pathToFileURL(filename),
    sourceMap: false,
    syntax: lang === 'sass' ? 'indented' : 'scss',
    importers: [internalImporter, ...(sassOptions.importers ?? [])],
  })

if you wanna change it to that, let me know (or feel free to edit yourself, if you prefer).

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commit: 8ccf5db

[sxzz]

Thanks!