READFROMFILE reads a number of bytes from the file depending on the mode parameter.
The start of the read process is controlled by the index parameter.

READFROMFILE(mode, index) => number/string

• mode = 0: character string, 0-terminated
• mode = 1: 1 byte as a natural number
  

• index = -1: the read process starts at the current position
• index = 0: the read process starts at the beginning
• index > -1: the read process starts at index