Update to modern peer dependency style

I'm admittedly new to npm/webpack stuff but my understanding is that this project currently uses an old style of peer dependencies. Modern webpack does a full static analysis of the code and expects everything it finds require()'d to be available. The impact of this is that, for example, even though I don't have markdown-it-mathjax3 enabled in either markdown-it-pandoc or panwriter, panwriter still complains that it's missing. It's possible to configure panwriter to ignore it, but that's a change to the panwriter config. Might as well fix it properly in this repo.

Start by inspecting this repository's npm dependency metadata and how its markdown-it plugins are declared, then compare that setup with webpack's static-analysis requirements. Done means panwriter no longer reports an unused markdown-it-mathjax3 dependency when that plugin is disabled, without requiring a panwriter configuration workaround.