Add support for `<sub>` and `<sup>`

[CosmicHorrorDev]

<sub> and <sup> seem to be one of the more common elements that we are missing support for (so they render as regular text instead). It would be good to add support although it will likely be a pretty difficult change

[philocalyst]

Would love to see this happen <3 -- If you can point me to a place in the codebase to work on, I could take a look?