[php-src] PR #24199: [RFC] Add CSV extension (ext/csv)

Pull Request: https://github.com/php/php-src/pull/24199 Author: damek24 ## Summary This draft PR proposes adding a new CSV extension (ext/csv) to PHP core. The extension is based on Gina Peter Banyard's girgias/csv project (BSD-3-Clause) and extends it with file and stream handling. ### Features - RFC 4180-compatible CSV parsing and serialization - Locale-independent parsing - Multibyte delimiters, enclosures, and EOL sequences - Strict and lax collection parsing - Lazy iteration over CSV files - Streaming CSV output - No proprietary escape character ### Implementation - Six functions and one class in the Csv\ namespace - 78 PHPT tests - Six upstream issues identified and fixed during the port ### RFC https://wiki.php.net/rfc/csv_extension **Status:** Draft - this PR is not intended for merging until the RFC has been discussed and accepted. ### Attribution Based on girgias/csv by Gina Peter Banyard and contributors, licensed under BSD-3-Clause. Parts of the implementation were developed with LLM assistance. I have reviewed the code and take full responsibility for the contribution. Feedback on API design, implementation details, and portability is welcome.