Feat: support eval directives/roles

[agoose77]

This adds support for the eval`expr` role and

:::{eval}expr
:::

:::{eval:figure} myplot
:name: myplot
My plot with a mean value of {eval}`mean`.
:::

directives.

We do this by parsing the expression, and expecting the renderer to transform the expression into rendered content. Currently this relies on jupyterlab-imarkdown to do this, which already supports these kinds of tokens.

jupyterlab-imarkdown is not the only way to render these tokens - it does a lot more in order to store the results in the cell attachments

I do add an install dependency for the Python package on this extension, but it is an extra for now.