Vendor lightningcss-napi as turbopack-lightningcss-napi
Replace the external lightningcss-napi dependency with a vendored copy at turbopack/crates/turbopack-lightningcss-napi. Source files are copied verbatim from the upstream lightningcss repository, include the upstream LICENSE, and are formatted with cargo fmt. Bump lightningcss from 1.0.0-alpha.70 to 1.0.0-alpha.71 to match the vendored napi crate.
