Feature: Add $SEEK_SET, $SEEK_CUR, $SEEK_END

[santorofer]

Since the values of SEEK_SET, SEEK_CUR and SEEK_END aren't standard, this adds a way to access the C macros from TDI. This is for use with the FSEEK() builtin, like so:

FSEEK(_file, *, $SEEK_END);

Each constant required a new opcode, which have been allocated in opcodes.csv