perf: speed up page rendering and jekyll build process (#2034)

- Ensure inline scripts execute after the DOM has fully loaded.
- Use Rollup to bundle the theme-mode and Mermaid scripts, reducing the number of Jekyll include snippets.

_javascript/modules/components/mermaid.js

@@ -0,0 +1,60 @@
+/**
+ * Mermaid-js loader
+ */
+
+const MERMAID = 'mermaid';
+const themeMapper = Theme.getThemeMapper('default', 'dark');
+
+function refreshTheme(event) {
+  if (event.source === window && event.data && event.data.id === Theme.ID) {
+    // Re-render the SVG › <https://github.com/mermaid-js/mermaid/issues/311#issuecomment-332557344>
+    const mermaidList = document.getElementsByClassName(MERMAID);
+
+    [...mermaidList].forEach((elem) => {
+      const svgCode = elem.previousSibling.children.item(0).innerHTML;
+      elem.textContent = svgCode;
+      elem.removeAttribute('data-processed');
+    });
+
+    const newTheme = themeMapper[Theme.visualState];
+
+    mermaid.initialize({ theme: newTheme });
+    mermaid.init(null, `.${MERMAID}`);
+  }
+}
+
+function setNode(elem) {
+  const svgCode = elem.textContent;
+  const backup = elem.parentElement;
+  backup.classList.add('d-none');
+  // Create mermaid node
+  const mermaid = document.createElement('pre');
+  mermaid.classList.add(MERMAID);
+  const text = document.createTextNode(svgCode);
+  mermaid.appendChild(text);
+  backup.after(mermaid);
+}
+
+export function loadMermaid() {
+  if (
+    typeof mermaid === 'undefined' ||
+    typeof mermaid.initialize !== 'function'
+  ) {
+    return;
+  }
+
+  const initTheme = themeMapper[Theme.visualState];
+
+  let mermaidConf = {
+    theme: initTheme
+  };
+
+  const basicList = document.getElementsByClassName('language-mermaid');
+  [...basicList].forEach(setNode);
+
+  mermaid.initialize(mermaidConf);
+
+  if (Theme.switchable) {
+    window.addEventListener('message', refreshTheme);
+  }
+}