Add support for converting PDFs to Remarkable bundles about remarks

[lucasrla]

Interesting idea. Couple of quick thoughts:

1. You haven't listed them explicitly but the JSON-based "smart" highlights might be the easiest way to starting building the PDF-to-bundle part. Those JSON files seem easy to create from scratch. Personally, I sometimes highlight PDFs on macOS Preview, so I could see the value of having my Preview highlights also available within the reMarkable tablet.

2. Apart from the reMarkable device, what are the other relevant ways remarks users (current or future) draw scribbles on documents? Would it be drawing with a mouse on something like macOS Preview (or Adobe Acrobat)? Maybe something with iPad + Apple Pencil? Knowing more about the nature of the scribbles to be written to RM files (and having actual examples to compare against) would be very helpful to define the scope of such a feature.

3. The RM parsing (coupled with some of the drawing) of remarks' codebase is messy. They were the very first parts that I quickly put together and I haven't touched them in years... They would certainly need some love (a complete rewrite?!) if we were to extend/build upon them.

In any case, happy to consider PRs related to your idea. Just let us know what you think!

Thanks