ISSUE #35: Structured Append Support

[cdm-phm]

fixes #35
This is a minimal PR to share some changes I needed to do for supporting structured append.

[manuelbl]

Thanks a lot for your PR. I'll have a closer look at it.

So far, the library has been 1 to 1 translation of https://github.com/nayuki/QR-Code-generator. This would be a deviation. Since it doesn't touch the core of the encoding, it might be worth it.

[cdm-phm]

I'm fully ok if this doesn't get integrated, I just used "your" library for a commercial/serious project and thought it is only fair to show the changes I needed.
As you seem to be Swiss you might be interested to know: printing medical bills now (from Jan 1st 2026) requires a QR-Page with the digital version of the bill encoded for easier scanning by the insurances, which employs the structured append mode.
This is the relevant "specification": https://www.forum-datenaustausch.ch/fileadmin/fda_documents/5.0/Rechnungen/Beispiele-XML-PDF/printTemplates_GIReq500_Annex.pdf