Fenced code blocks does not check for backticks

[susnux]

Fenced code blocks should check the content for any sequence of backticks, as CommonMark defines a code block to end with a sequence of backticks of at least the number it started.

Meaning this is perfectly valid markdown:

code

But currently prosemirror-markdown will serialize this:

<pre><code>

code

</code></pre>

to this markdown:

code

which is a code block followed by a paragraph "code" and a second code block.