Add elk layout support to mermaid (#36486)

Fixes: https://github.com/go-gitea/gitea/issues/34769

This allows the user to opt-in to using `elk` layouts using either YAML
frontmatter or `%%{ init` directives inside the markup code block. The
default layout is not changed.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/utils/testhelper.ts

@@ -4,3 +4,19 @@
 export function isInFrontendUnitTest() {
   return import.meta.env.TEST === 'true';
 }
+
+/** strip common indentation from a string and trim it */
+export function dedent(str: string) {
+  const match = str.match(/^[ \t]*(?=\S)/gm);
+  if (!match) return str;
+
+  let minIndent = Number.POSITIVE_INFINITY;
+  for (const indent of match) {
+    minIndent = Math.min(minIndent, indent.length);
+  }
+  if (minIndent === 0 || minIndent === Number.POSITIVE_INFINITY) {
+    return str;
+  }
+
+  return str.replace(new RegExp(`^[ \\t]{${minIndent}}`, 'gm'), '').trim();
+}