[FR] Pandoc 2.19+ allows to opt-out mathml in epub3 format and choose other math method too

[cderv]

This follows up on #1417 where we only brought back previous bookdown behavior on epub_book() following Pandoc 2.19 change.

We can probably add more feature to add a math argument and allow to opt-out

For now epub_book() only use mathml by default and it seems to works ok for bookdown users.