feat: expand CSV reader/writer with config, delimiters, and tests (#41)

- Add CsvConfig struct for customizable headers, delimiter, and flexibility
- Add from_str_with_config() and from_reader_with_config() for custom parsing
- Add to_string_with_config() for custom serialization
- Support no-header mode (array of arrays)
- Support custom delimiters (tab, pipe, semicolon, etc.)
- Support flexible/ragged rows with extra column mapping
- Union headers from all rows when serializing
- Support writing array-of-arrays without headers
- Add 45+ tests covering: basic CSV, no-header mode, TSV/pipe/semicolon
  delimiters, quoted fields (commas/quotes/newlines), type inference,
  empty fields, cross-format roundtrips (CSV↔JSON), large CSV (1000 rows),
  unicode headers/values, ragged rows (strict + flexible), union headers,
  null serialization, bool roundtrip, header order, whitespace handling,
  array-of-arrays output, and no-header roundtrip

Fixes #11

Author: alvinreal