Seek the stream. seekCurrent seeks from the current stream position, seekAboslute seeks to the given position offset from the beginning of the stream, and seekEnd seeks to the position offset bytes from the end of the stream (backwards).
ulong seekEnd (
Note, this throws an exception if seeking fails or isn't supported.
offset = bytes to seek.
The position of the stream from the beginning of the stream after seeking, or ulong.max if this cannot be determined.