feat(turbopack): add LocalPathOrProjectPath PostCSS config resolution
Add a new PostCssConfigLocation::LocalPathOrProjectPath variant that
searches for postcss config starting from the CSS file's parent directory
first, then falls back to the project root. This allows per-directory
postcss.config.js files to override the project root config.
Replace ProjectPathOrLocalPath with LocalPathOrProjectPath in both client
and server contexts so user code benefits from local-first config
resolution. Add doc comments to all PostCssConfigLocation variants.
https://claude.ai/code/session_01VjFd6129z6XbursoXTf9g7