function greyscale (greyscale) {
  this.options.greyscale = is.bool(greyscale) ? greyscale : true;
  return this;
}
Convert to 8-bit greyscale; 256 shades of grey.
This is a linear operation. If the input image is in a non-linear colour space such as sRGB, use `gamma()` with `greyscale()` for the best results.
By default the output image will be web-friendly sRGB and contain three (identical) colour channels.
This may be overridden by other sharp operations such as `toColourspace('b-w')`,
which will produce an output image containing one colour channel.
An alpha channel may be present, and will be unchanged by the operation.

@example
const output = await sharp(input).greyscale().toBuffer();

@param {Boolean} [greyscale=true]
@returns {Sharp}