Add cell-id anchor for cell identification

[krassowski]

Addresses #1862.

Reference implementation for jupyter/nbformat#317. Accompanies a PR for JupyterLab: jupyterlab/jupyterlab#13285.

Adds a id attribute to code and markdown cells for cells which have ID, which contains cell id prefixed with cell-id.

Code cell:

<div id="cell-id=a91264b6-ccda-4dad-ab2e-60bcc12c0066" class="jp-Cell jp-CodeCell jp-Notebook-cell jp-mod-noOutputs">
    <div class="jp-Cell-inputWrapper">
        <div class="jp-Collapser jp-InputCollapser jp-Cell-inputCollapser"></div>
        <div class="jp-InputArea jp-Cell-inputArea">
            <div class="jp-InputPrompt jp-InputArea-prompt">In&nbsp;[&nbsp;]:</div>
            <div class="jp-CodeMirrorEditor jp-Editor jp-InputArea-editor" data-type="inline">
                <div class="CodeMirror cm-s-jupyter">
                    <div class="highlight hl-ipython3">
                        <pre><span></span><span class="n">test</span></pre>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

Markdown:

<div id="cell-id=0df699d1-77ea-432a-a203-fafd77e87462" class="jp-Cell jp-MarkdownCell jp-Notebook-cell">
    <div class="jp-Cell-inputWrapper">
        <div class="jp-Collapser jp-InputCollapser jp-Cell-inputCollapser"></div>
        <div class="jp-InputArea jp-Cell-inputArea">
            <div class="jp-InputPrompt jp-InputArea-prompt"></div>
            <div class="jp-RenderedHTMLCommon jp-RenderedMarkdown jp-MarkdownOutput" data-mime-type="text/markdown">
                <p>test</p>
            </div>
        </div>
    </div>
</div>

Question: why are cell tags for markdown cells in lab template applied on jp-MarkdownOutput rather than for jp-MarkdownCell, differently to code cells and classic template?

[ndmlny-qs]

lgtm