[Code Blocks] Allow <pre> to contain <code> child

[seshrs]

Context

While browsing EECS 183 Project 1, I saw the following code block that had a differently-styled first-line:

<pre><code>if (loaves == 1) {
    //do something
} else if (loaves == 2) {
    //do something else
} // and so on</code></pre>

I had not anticipated that a manual <pre> block might also contain a single <code> block child. This commit modifies the code-block detection algorithm to look ignore the inner <code> block if applicable.

Validation

Before	After

[github-actions]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/147/.

^{(Available until Thu Feb 10 2022.)}