Always suffix indented code block with a newline

[hukkin]

Currently

    codeblock\n

(where \n represents a trailing newline) renders differently to

    codeblock

(no trailing newline).

The former renders to

<pre><code>codeblock
</code></pre>

and the latter to

<pre><code>codeblock</code></pre>

This PR makes the two examples render to the same HTML by always adding a trailing newline to code block content.

The spec is not overly clear about this corner case, but this PR makes

• the behavior in line with how the reference implementation parses and renders https://spec.commonmark.org/dingus/?text=%20%20%20%20codeblock
• makes it so that any indented code block can be reformatted as code fence (useful for markdown formatters)

[rlidwka]

Merged, thanks!

Is code block the only place where the absense of trailing newline in original input matters?

[hukkin]

Is code block the only place where the absense of trailing newline in original input matters?

This I don't have an answer to right now, I haven't tested. I found this issue while fuzzing mdformat and if there are other similar issues with other syntax then the fuzzer should find them when I have the time to get back into it.