Diagrams do not appear as images, but as source code

[floscher]

When a code block has a language starting with diagram-, then the diagram should be rendered as image in the resulting HTML.

This already worked previously (e.g. this blogpost), but probably with an update of marked this was broken.

Example

This source code in a blogpost:

```diagram-mermaid
graph TD
  A --> B
```

Currently renders like this:

<svg aria-roledescription="flowchart-v2" role="graphics-document document" viewBox="-8 -8 40.734375 134" height="134" xmlns="http://www.w3.org/2000/svg" width="40.734375" id="container"><style>#container{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}#container .error-icon{fill:#552222;}#container .error-text{fill:#552222;stroke:#552222;}#container .edge-thickness-normal{stroke-width:2px;}#container .edge-thickness-thick{stroke-width:3.5px;}#container .edge-pattern-solid{stroke-dasharray:0;}#container .edge-pattern-dashed{stroke-dasharray:3;}#container .edge-pattern-dotted{stroke-dasharray:2;}#container .marker{fill:#333333;stroke:#333333;}#container .marker.cross{stroke:#333333;}#container svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#container .label{font-family:"trebuchet ms",verdana,arial,sans-serif;color:#333;}#container .cluster-label text{fill:#333;}#container .cluster-label span,#container p{color:#333;}#container .label text,#container span,#container p{fill:#333;color:#333;}#container .node rect,#container .node circle,#container .node ellipse,#container .node polygon,#container .node path{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#container .flowchart-label text{text-anchor:middle;}#container .node .katex path{fill:#000;stroke:#000;stroke-width:1px;}#container .node .label{text-align:center;}#container .node.clickable{cursor:pointer;}#container .arrowheadPath{fill:#333333;}#container .edgePath .path{stroke:#333333;stroke-width:2.0px;}#container .flowchart-link{stroke:#333333;fill:none;}#container .edgeLabel{background-color:#e8e8e8;text-align:center;}#container .edgeLabel rect{opacity:0.5;background-color:#e8e8e8;fill:#e8e8e8;}#container .labelBkg{background-color:rgba(232, 232, 232, 0.5);}#container .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;}#container .cluster text{fill:#333;}#container .cluster span,#container p{color:#333;}#container div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33;border-radius:2px;pointer-events:none;z-index:100;}#container .flowchartTitleText{text-anchor:middle;font-size:18px;fill:#333;}#container :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;}</style><g><marker orient="auto" markerHeight="12" markerWidth="12" markerUnits="userSpaceOnUse" refY="5" refX="6" viewBox="0 0 10 10" class="marker flowchart" id="container_flowchart-pointEnd"><path style="stroke-width: 1; stroke-dasharray: 1, 0;" class="arrowMarkerPath" d="M 0 0 L 10 5 L 0 10 z"></path></marker><marker orient="auto" markerHeight="12" markerWidth="12" markerUnits="userSpaceOnUse" refY="5" refX="4.5" viewBox="0 0 10 10" class="marker flowchart" id="container_flowchart-pointStart"><path style="stroke-width: 1; stroke-dasharray: 1, 0;" class="arrowMarkerPath" d="M 0 5 L 10 10 L 10 0 z"></path></marker><marker orient="auto" markerHeight="11" markerWidth="11" markerUnits="userSpaceOnUse" refY="5" refX="11" viewBox="0 0 10 10" class="marker flowchart" id="container_flowchart-circleEnd"><circle style="stroke-width: 1; stroke-dasharray: 1, 0;" class="arrowMarkerPath" r="5" cy="5" cx="5"></circle></marker><marker orient="auto" markerHeight="11" markerWidth="11" markerUnits="userSpaceOnUse" refY="5" refX="-1" viewBox="0 0 10 10" class="marker flowchart" id="container_flowchart-circleStart"><circle style="stroke-width: 1; stroke-dasharray: 1, 0;" class="arrowMarkerPath" r="5" cy="5" cx="5"></circle></marker><marker orient="auto" markerHeight="11" markerWidth="11" markerUnits="userSpaceOnUse" refY="5.2" refX="12" viewBox="0 0 11 11" class="marker cross flowchart" id="container_flowchart-crossEnd"><path style="stroke-width: 2; stroke-dasharray: 1, 0;" class="arrowMarkerPath" d="M 1,1 l 9,9 M 10,1 l -9,9"></path></marker><marker orient="auto" markerHeight="11" markerWidth="11" markerUnits="userSpaceOnUse" refY="5.2" refX="-1" viewBox="0 0 11 11" class="marker cross flowchart" id="container_flowchart-crossStart"><path style="stroke-width: 2; stroke-dasharray: 1, 0;" class="arrowMarkerPath" d="M 1,1 l 9,9 M 10,1 l -9,9"></path></marker><g class="root"><g class="clusters"></g><g class="edgePaths"><path marker-end="url(#container_flowchart-pointEnd)" style="fill:none;" class="edge-thickness-normal edge-pattern-solid flowchart-link LS-A LE-B" id="L-A-B-0" d="M12.367,34L12.367,38.167C12.367,42.333,12.367,50.667,12.367,58.117C12.367,65.567,12.367,72.133,12.367,75.417L12.367,78.7"></path></g><g class="edgeLabels"><g class="edgeLabel"><g transform="translate(0, 0)" class="label"><foreignObject height="0" width="0"><div style="display: inline-block; white-space: nowrap;" xmlns="http://www.w3.org/1999/xhtml"><span class="edgeLabel"></span></div></foreignObject></g></g></g><g class="nodes"><g transform="translate(12.3671875, 17)" data-id="A" data-node="true" id="flowchart-A-0" class="node default default flowchart-label"><rect height="34" width="24.171875" y="-17" x="-12.0859375" ry="0" rx="0" style="" class="basic label-container"></rect><g transform="translate(-4.5859375, -9.5)" style="" class="label"><rect></rect><foreignObject height="19" width="9.171875"><div style="display: inline-block; white-space: nowrap;" xmlns="http://www.w3.org/1999/xhtml"><span class="nodeLabel">A</span></div></foreignObject></g></g><g transform="translate(12.3671875, 101)" data-id="B" data-node="true" id="flowchart-B-1" class="node default default flowchart-label"><rect height="34" width="24.734375" y="-17" x="-12.3671875" ry="0" rx="0" style="" class="basic label-container"></rect><g transform="translate(-4.8671875, -9.5)" style="" class="label"><rect></rect><foreignObject height="19" width="9.734375"><div style="display: inline-block; white-space: nowrap;" xmlns="http://www.w3.org/1999/xhtml"><span class="nodeLabel">B</span></div></foreignObject></g></g></g></g></g></svg>

But instead should render like this:

graph TD
  A --> B