Decoupling plain placeholder from pdfkit010

[vbuch]

Decoupling plainAddPlaceholder from pdfkit010 should be something very easy to do. I think we only use pdfkit for keeping track of references and encoding PDFObject. It is a task that Copilot should probably easily manage.

Note that there already is /packages/placeholder-pdfkit/src/pdfkit/pdfobject.js. Maybe move it to utils? And then PDFKitReferenceMock doesn't seem to do much.