Revisit math mode support in Markdown

[caizixian]

Currently, math mode is completely implemented in the frontend via marked.

notegraf/notegraf-web/src/frontend/components/Note.tsx

Line 58 in 4d261bb

const renderer = {

pulldown-cmark https://github.com/pulldown-cmark/pulldown-cmark/releases/tag/v0.11.0 added math mode support, and it would be nice that the frontend supports something compatible.

pulldown-cmark is not currently used, because pulldown-cmark-to-cmark needs to be ported to support the new version of pulldown-cmark Byron/pulldown-cmark-to-cmark#72