export: add pdf

[gantrol]

试了一下，比想象中要难得多，暂时搁置。如有可能，会跟“重构图片导出”一起做。计划使用PDF.js实现。

I gave it a try, and it's much harder than imagined, so I'm putting it on hold for now. If possible, I'll work on it together with "Refactor image export".