splitOption() discards description text after delimiter

Problem/Motivation

WebformOptionsHelper::splitOption() currently splits on all occurrences of the delimiter " -- ", but callers consume only the first two items. If a description contains the phrase " -- ", any text after it will be discarded.

Proposed resolution

Descriptions can be allowed to contain " -- " by passing a limit to explode():

  public static function splitOption($text) {
    return explode(static::DESCRIPTION_DELIMITER, $text, 2);
  }