🥩 Add raw directive

[fwkoch]

The raw directive lets you include raw latex in a myst file. The latex will be parsed using tex-to-myst:

```{raw} latex
\section{Hello World!}
\textbf{Hello World!} Here's some latex!
\begin{equation}
\gamma^2+\theta^2=\omega^2
\end{equation}
Pretty cool!
```

You can also use the raw directive to add unparsed text:

```{raw}
*Keep these astrisks!*
```

[fwkoch]

I think there's more to do on this, in terms of getting this into static exports correctly, supporting html and other raw types, maybe combining logic with include - since raw is similar to include, the content is just embedded in the page...

For now though, i'll land this first pass!