Fix unicode support in appendSlowly method

[overlord-space]

Currently, if we use for example Cyrillic symbols in typeSlowly or appendSlowly methods, they are processed with str_split function, which does not correctly processes unicode characters.

str_split('Образец')

will return:

array:14 [
  0 => b"Ð"
  1 => b"ž"
  2 => b"Ð"
  3 => b"±"
  4 => b"Ñ"
  5 => b"€"
  6 => b"Ð"
  7 => b"°"
  8 => b"Ð"
  9 => b"·"
  10 => b"Ð"
  11 => b"µ"
  12 => b"Ñ"
  13 => b"†"
]

After that, web-driver will send the sendKeysToElement command with parameters similar as follows: "�" instead of {"text":"О",":id": ...}

Server will respond to this request with the following exception: "invalid argument: missing command parameters".

Replacing str_split with mb_str_split (PHP >= 7.4) or with preg_split('\\u', ...) fix this issue.

preg_split('//u', 'Образец', -1, PREG_SPLIT_NO_EMPTY)

=> array:7 [
  0 => "О"
  1 => "б"
  2 => "р"
  3 => "а"
  4 => "з"
  5 => "е"
  6 => "ц"
]

And server will receive correct command parameters:
sendKeysToElement || {"text":"\u0430",":id": ...}