Improve PDFPC support

[Beiri22]

I just saw that you now support pdfpc, which is really nice. I thought if the intermediate step of processing the query output could be avoided. Would it be possible to save the metadata of each slide into a state at first; and create some hook to emit the #metadata at the end of each slide in exactly the format that pdfpc requires?

I'm experimenting with a different approach for another 3rd party integration right now.

[andreasKroepelin]

I think it is a bit cumbersome to massage the metadata and typst query into emitting exactly the format expected by pdfpc... Earlier or later you would need some external tool anyways and then I prefer to have the complex code on the Rust side and the Typst code as simple as possible.

[Beiri22]

I'm not 100% convinced yet. Just to have a look myself, do you know where the pdfpc format is documented?

[andreasKroepelin]

It is documented here:
https://github.com/pdfpc/pdfpc/blob/master/schema/pdfpc.json