ID of the `div.highlight pre` element gets overwritten

[So-Cool]

Describe the bug

The extension overwrites the id of the div.highlight pre element with codecell# (if it already has one). The expected behaviour would be to use the id that already exists.

To Reproduce

1. Create a code block with an id, e.g.:

   <div class="highlight">
      <pre id="my-id">
          code...
      </pre>
   </div>

2. Open the HTML page.
3. my-id is overwritten with codecell0.

Expected behavior

my-id is preserved and used to facilitate the extension functionality.

Environment

• Python Version [e.g. 3.7.1]: 3.8.2
• Package versions or output of jupyter-book --version: 0.3.1
• Operating System: MacOS 10.15.6

Additional context

Jupyter Book: 0.9.1
MyST-NB: 0.10.1
Sphinx Book Theme: 0.0.39
MyST-Parser: 0.12.10
Jupyter-Cache: 0.4.1
NbClient: 0.5.1

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