SCSS/SASS port of PHP function substr()

_substr.scss

/// Returns part of string.
/// Similar to PHP `substr()`: `substr("abc", 2, 1)` - "c", `substr("abc", 1)` - "bc", `substr("abcd", -3, -1)` - "bc".
///
/// @param {String} $str The input string
/// @param {Number} $offset The returned string will start at the offset'th position in string, counting from zero
/// @param {Number} $length The string returned will contain at most length characters beginning from offset (optional)
/// @return {String}
/// @see https://www.php.net/manual/en/function.substr
@function substr($str, $offset, $length: 0) {
  $length: if($length < 0, $length - 1, if($length == 0, str-length($str), $length));
  @return str-slice(str-slice($str, $offset), 1, $length);
}