Support custom EOL character when encoding CSV

[clue]

This changeset adds support for specifying a custom EOL character. This can be useful when creating a CSV file using the Encoder for Windows using the CRLF sequence (\r\n) like this:

$encoder = newClue\React\Csv\Encoder($this->output, ',', '"', '\\', "\r\n");

This is supported natively by PHP as of PHP 8.1, but this PR also implements the same feature for all supported PHP versions.

Refs https://www.php.net/manual/en/migration81.new-features.php#migration81.new-features.standard