Add support for PDF rendering

[kkoreilly]

Describe the feature

We should add support for rendering PDFs at some point, which will be useful generally and for apps like Cogent Mail and Cogent Author. We could either render PDFs as images, render them as Cogent Core widgets, or convert them to HTML and then render them through htmlcore. We will also need to create PDFs in Cogent Canvas, Cogent Docs, Cogent Slides, etc at some point. Potentially relevant packages:

• https://github.com/gen2brain/go-fitz
• https://github.com/pdfcpu/pdfcpu
• https://github.com/unidoc/unipdf
• https://github.com/dslipak/pdf
• https://github.com/go-pdf/fpdf
• https://github.com/sajari/docconv
• https://github.com/adrg/go-wkhtmltopdf

The only packages with full functionality use CGO (like go-fitz above).

Relevant code

No response