[FR] Allow to use $$ for numbering equation in **bookdown**

[cderv]

This based on ideas and discussion with @Abhi-1U around his work on texor, and what we do in Quarto.

We can use Lua to tweak the rendering and so remove some contraint.

For numering equation, this is documented here: https://bookdown.org/yihui/bookdown/markdown-extensions-by-bookdown.html#equations

To number and refer to equations, put them in the equation environments and assign labels to them using the syntax (#eq:label)

Currently it needs to be inside \begin{equation} environment, and can't be inside usual $$. Though we can catch the DisplayMath, and emit the right LaTeX we need.

See this self contained reprex in this gist https://gist.github.com/cderv/13b667d1c770fb29ba1fb28cc43f228b

• HTML

• PDF

We could rethink the syntax also. Quarto does a specific new syntax
https://quarto.org/docs/authoring/cross-references.html#equations