Use a generic markup class to display externally rendered files and diffs (#15735)

* creates and implements generic markup less class

* How to give custom CSS to externally rendered html

* Clarifies sources of CSS styling of markup

* further clarification of sources of markup styling

* rename _markdown to _markup

* remove defunct import

* fix orphaned reference

* Update docs/content/doc/advanced/external-renderers.en-us.md

* more renames markdown -> markup

* do not suggest less customization

* add back tokens

* fix class whitespace, remove useless if-clause

* remove unused csv-data rules

* use named exports and rename functions

* sort imports

Co-authored-by: HarvsG <11440490+HarvsG@users.noreply.github.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: silverwind <me@silverwind.io>

web_src/js/markup/mermaid.js

@@ -0,0 +1,56 @@
+const MAX_SOURCE_CHARACTERS = 5000;
+
+function displayError(el, err) {
+  el.closest('pre').classList.remove('is-loading');
+  const errorNode = document.createElement('div');
+  errorNode.setAttribute('class', 'ui message error markup-block-error mono');
+  errorNode.textContent = err.str || err.message || String(err);
+  el.closest('pre').before(errorNode);
+}
+
+export async function renderMermaid(els) {
+  if (!els || !els.length) return;
+
+  const mermaid = await import(/* webpackChunkName: "mermaid" */'mermaid');
+
+  mermaid.initialize({
+    mermaid: {
+      startOnLoad: false,
+    },
+    flowchart: {
+      useMaxWidth: true,
+      htmlLabels: false,
+    },
+    theme: 'neutral',
+    securityLevel: 'strict',
+  });
+
+  for (const el of els) {
+    if (el.textContent.length > MAX_SOURCE_CHARACTERS) {
+      displayError(el, new Error(`Mermaid source of ${el.textContent.length} characters exceeds the maximum allowed length of ${MAX_SOURCE_CHARACTERS}.`));
+      continue;
+    }
+
+    let valid;
+    try {
+      valid = mermaid.parse(el.textContent);
+    } catch (err) {
+      displayError(el, err);
+    }
+
+    if (!valid) {
+      el.closest('pre').classList.remove('is-loading');
+      continue;
+    }
+
+    try {
+      mermaid.init(undefined, el, (id) => {
+        const svg = document.getElementById(id);
+        svg.classList.add('mermaid-chart');
+        svg.closest('pre').replaceWith(svg);
+      });
+    } catch (err) {
+      displayError(el, err);
+    }
+  }
+}