enhance: update mermaid to v12 (#39331)

Update to [mermaid
v12](https://github.com/mermaid-js/mermaid/releases#release-mermaid@12.0.0),
ELK is now the default layout so the plugin and all supporting code is
gone.

Layout switching to previous `dagre` layout via frontmatter works as
expected. Diagrams use the new `neo` default look which renders them
slightly different (smaller boxes) then the `classic` look that was
default in v11.

The `neo` look got some tweaks to remove shadows and gradient to look
better. Also did some related refactors and mermaid dragging now works
on touch devices too.

web_src/js/utils/testhelper.ts

@@ -12,22 +12,6 @@ export function captureNavigations() {
   return navigations;
 }
 
-/** 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 = Infinity;
-  for (const indent of match) {
-    minIndent = Math.min(minIndent, indent.length);
-  }
-  if (minIndent === 0 || minIndent === Infinity) {
-    return str;
-  }
-
-  return str.replace(new RegExp(`^[ \\t]{${minIndent}}`, 'gm'), '').trim();
-}
-
 export function normalizeTestHtml(s: string) {
   const lines = s.replace(/>\s+</g, '>\n<').trim().split('\n');
   for (let i = 0; i < lines.length; i++) {