layouts: add Mermaid support

Similar to Graphviz and TikZ support.

layouts/partials/head-extra.html

@@ -20,6 +20,13 @@
     })();
     </script>
 {{ end }}
+<!-- Mermaid support -->
+{{ if (.Params.mermaid) }}
+    <script type="module" async>
+        import mermaid from "https://cdn.jsdelivr.net/npm/mermaid@latest/dist/mermaid.esm.min.mjs";
+        mermaid.initialize({ startOnLoad: true });
+    </script>
+{{ end }}
 {{ with .OutputFormats.Get "atom" -}}
     {{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}
 {{ end -}}

layouts/shortcodes/mermaid.html

@@ -0,0 +1,16 @@
+<pre class="mermaid">
+    {{ with .Get "file" }}
+        {{ if eq (. | printf "%.1s") "/" }}
+            {{/* Absolute path are from root of site. */}}
+            {{ $.Scratch.Set "filepath" . }}
+        {{ else }}
+            {{/* Relative paths are from page directory. */}}
+            {{ $.Scratch.Set "filepath" $.Page.File.Dir }}
+            {{ $.Scratch.Add "filepath" . }}
+        {{ end }}
+
+        {{ $.Scratch.Get "filepath" | readFile }}
+    {{ else }}
+        {{.Inner}}
+    {{ end }}
+</pre>