Extracts part of the input text with the length
length from the input
text, starting with the character at
position is negative, it is counted from the end.
(Note: The numbering of the characters starts with 0, i.e.
position specifies how many characters are to be
textAtPosition('ene mene muh', 4, 2)→ ‘me’.
textAtPosition('ene mene muh', -3, 2)→ ‘mu’.