Implement the File API's File.seek method

[oleiade]

As part of #3141, implementing the agreed-upon design for k6's File API, we intend to implement a File.seek method.

The seek method seeks the file to handle to the given offset under the mode provided by whence. The call resolves with the new position within the resource (offset from the start, in bytes).

prototype

interface File {
    seek(offset: number, whence: SeekMode): Promise<number>
}

enum SeekMode {
    Current = 1
    End = 2
    Start = 0
}

example

const file = fs.openSync("fou.txt")
const cursorPosition = await file.seek(6, fs.SeekMode.Start)

[oleiade]

Implemented by #3309