Clear and concise description of the problem

tsdown should be able to transform CSS to lower targets, like what tsup does.

Reproduction: https://github.com/issueset/test-tsdown-style-trans

Expected output (dist/output_tsup/style.css):

/* src/style.css */
.style .inner-a {
  border-color: red;
}
.style .inner-b {
  border-color: blue;
}

Current output (dist/output_tsdown/style.css):

.style {
  & .inner-a {
    border-color: red;
  }
  & .inner-b {
    border-color: blue;
  }
}

Suggested solution

Use unplugin-lightningcss to implement CSS transformation, until rolldown itself has enough bandwidth for CSS support (see rolldown/rolldown#4271).

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.