Compatibility with micromark's nested directive syntax

[dmca-glasgow]

Describe your proposed improvement and the problem it solves.

Given the following LaTeX:

\begin{task}
Test task
  \begin{answer}
  Test answer
  \end{answer}
\end{task}

When running the command:

pandoc --output test.md test.tex

The resulting Markdown looks like this:

::: task
Test task

::: answer
Test answer
:::
:::

In order to be comaptible with micromark-extension-directive, Pandoc could match their nested directive syntax, which requires outer directives to increment preceding colons (search their readme for "nest" to see their example), for example:

:::: task
Test task

::: answer
Test answer
:::
::::

Thanks.

Related discussion: #9449