Added fs.FileHandle.

Added fs.FileHandle.

The following methods are implemented:
    - fs.openSync(path[, flag[, mode]])
    - fs.promises.open(path[, flag[, mode]])
    - fs.fstatSync(fd)
    - fs.readSync(fd, buffer, offset[, length[, position]])
    - fs.writeSync(fd, buffer, offset[, length[, position]])
    - fs.writeSync(fd, string[, position[, encoding]])

The following properties of FileHandle are implemented:
    - filehandle.fd
    - filehandle.read(buffer, offset, [length[, position]])
    - filehandle.stat()
    - filehandle.write(buffer, offset, [length[, position]])
    - filehandle.write(string[, position[, encoding]])
    - filehandle.close()