Fenced div separator fix

[lostenderman]

[fix in progress]

A top_level group is added modifying whether a fencestart (:::) can interrupt a paragraph or not. Two versions of EndlineExceptions are therefore used, one with a fencestart (when is_top_level is true) and one without. The is_top_level group is set to false before parsing a V("Block") and set to true before parsing a (V("Paragraph").

Passes:

::: {#div}
> a
> :::

b
:::

Fails:

::: {#div}
> a
:::

[Witiko]

Closed via Witiko#431.