Do not end paragraph before ::: in fenced divs

[Witiko]

This PR is a version of lostenderman#157, which was supposed to close #407, with the correct base repository set.

[Witiko]

@lostenderman Upon a review of your changes and investigating the alternative route in #428, your PR still seems as the preferred way to go. It seems that we are almost there, we just need a way to make fencestart on the last line of a blockquote / list end the blockquote / list:

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

Of course, we only want this with top-level blockquotes, which makes things difficult. I am not clear on how we should approach this. At this point, it may be useful to brainstorm ideas.

[Witiko]

Closed via #431.