Link definitions not propogated to nested parser

Currently, link definitions cannot be used inside directives, e.g.

```{note}
[reference]
```

[reference]: https://github.com/ExecutableBookProject

will not work, but this will:

```{note}
[reference]

[reference]: https://github.com/ExecutableBookProject
```

This will also be the case for upcoming footnotes

[chrisjsewell]

Actually I’m not sure if this is strictly part of the CommonMark spec (see this interactive demo). I’ll have to see in mistletoe-ebp, if storing all potential definition reference as ‘pending references’ breaks any tests, because this would make things a little easier.

[chrisjsewell]

I’m not sure of an ideal solution off-hand; just somehow you need to ‘defer’ the processing of Markdown span tokens until you’ve found all possible link/footnote definitions.

It’s a bit of a peculiarity of Markdown, that for example, you can’t just identify the references through a regex, then store them for later replacement with the definitions, because whether or not they can be immediately resolved can have a direct bearing on how the subsequent text is parsed.

For example, this:

[a*b]*

[a*b]: asd

Is parsed to this

a*b*

but if the definition is not available:

[c*d]*

[cd]