Improve additional issue source formatting and use file_path for internal checks
- Add file_path field to PlainSource/NapiSource for proper path-based filtering
- Use formatFilePath for additional source paths instead of raw idents
- Skip code frames for internal/bundled files based on file_path
- Prefix unmapped sources with "Generated code of" for clarity
- Fix column offset in turbopack-cli-utils issue formatting
- Update sass-error test snapshot to include PostCSS transform context