Properly expose image conversion options

[Cellivar]

Turns out I forgot to do this. This is a breaking change.

This moves the image dithering options directly into the image options object that can be passed to addImageFromImageData, the idea being the consumer of the library can alter the conversion options to suit their needs depending on their printer's shenanigans.

This also alters the default gray cutoff down to 25% instead of 75%, mostly to see if that improves things.

[github-actions]

➿ Code coverage

Filename	Statements	Branches	Functions	Lines
src/Documents/BitmapGRF.ts	53.28%	44.83%	68.75%	55.37%
src/WebZlpError.ts	33.33%	100%	0%	33.33%
src/index.ts	100%	100%	100%	100%
src/Documents/Commands.ts	37.5%	43.33%	7.59%	37.36%
src/Printers/Configuration/PrinterOptions.ts	28.95%	66.67%	7.14%	28.95%
src/Printers/Models/PrinterModel.ts	42.86%	22.22%	6.67%	42.86%
src/Printers/Configuration/SerialPortSettings.ts	100%	100%	100%	100%
src/Printers/Configuration/MediaOptions.ts	70.77%	35.71%	71.43%	70.77%
src/Documents/ConfigDocument.ts	37.5%	0%	33.33%	37.5%
src/Documents/Document.ts	50%	0%	33.33%	55%
src/Documents/LabelDocument.ts	52%	66.67%	25%	52%
src/Documents/ReadyToPrintDocuments.ts	31.82%	100%	0%	33.33%
src/Printers/Communication/LineBreakTransformer.ts	12.5%	100%	0%	14.29%
src/Printers/Communication/PrinterCommunication.ts	84.62%	66.67%	66.67%	84.62%
src/Printers/Communication/UsbPrinterDeviceChannel.ts	7.04%	0%	0%	7.04%
src/Printers/Languages/EplPrinterCommandSet.ts	14.29%	8.47%	9.68%	14.97%
src/Printers/Models/PrinterModelDb.ts	18.52%	0%	0%	18.52%
src/Printers/Models/EplPrinterModels.ts	46.15%	100%	0%	46.15%
src/Printers/Languages/PrinterCommandSet.ts	22.45%	6.25%	12%	23.16%
src/Printers/PrinterCommunicationOptions.ts	25%	100%	100%	25%
src/Printers/Printer.ts	11.43%	0%	0%	11.54%
src/Printers/Languages/ZplPrinterCommandSet.ts	4.35%	0%	0%	4.38%
src/PrinterUsbManager.ts	6.52%	0%	0%	6.82%