✨ NEW: Add math directive

[rowanc1]

Introduce a math directive:

```{math}
:label: my_label
w_{t+1} = (1 + r_{t+1}) s(w_t) + y_{t+1}

Development:
* Better typing for options

Leaving out of this PR:
* Actually dealing with the labels

[chrisjsewell]

This obviously ties in to https://github.com/executablebooks/markdown-it-amsmath/issues/3, i.e. I think here there needs to be a consistent way to have this math rendered by e.g. katex, within the markdown-it parse/render process

[rowanc1]

This will need another pass to reconcile with the labeling and the inline math renderer. Once we get it in, can do another pass on that before releasing.

[rowanc1]

Let's get this in, we can do any reconciliation with other math nodes later, the only change would be to the options which is in another branch.