Add support for in-line asides

[agoose77]

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Asides at present are block elements that interrupt the authoring flow, e.g.

Hello world. 
:::{aside}
This is an aside
:::
Goodbye moon

We should make it possible to define an aside out of the current block, and link to it, e.g.

Hello world.[^note] Goodbye moon

:::{aside}
:name: note
This is an aside
:::

There are many ways we could implement this syntax. c.f. https://hackmd.io/Q_to9V1cS-2Q0RGEFFIm7w

For example, this approach imposes the aside-vs-footnote choice on the reference not the definition:

Hello world.{aside}`note` Goodbye moon

[^note]: This is an aside

I prefer (1) as it avoids duplicate definitions.