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/anchors.js

@@ -0,0 +1,32 @@
+import {svg} from '../svg.js';
+
+const headingSelector = '.markup h1, .markup h2, .markup h3, .markup h4, .markup h5, .markup h6';
+
+function scrollToAnchor() {
+  if (document.querySelector(':target')) return;
+  if (!window.location.hash || window.location.hash.length <= 1) return;
+  const id = decodeURIComponent(window.location.hash.substring(1));
+  const el = document.getElementById(`user-content-${id}`);
+  if (el) {
+    el.scrollIntoView();
+  } else if (id.startsWith('user-content-')) { // compat for links with old 'user-content-' prefixed hashes
+    const el = document.getElementById(id);
+    if (el) el.scrollIntoView();
+  }
+}
+
+export function initMarkupAnchors() {
+  if (!document.querySelector('.markup')) return;
+
+  for (const heading of document.querySelectorAll(headingSelector)) {
+    const originalId = heading.id.replace(/^user-content-/, '');
+    const a = document.createElement('a');
+    a.classList.add('anchor');
+    a.setAttribute('href', `#${encodeURIComponent(originalId)}`);
+    a.innerHTML = svg('octicon-link');
+    heading.prepend(a);
+  }
+
+  scrollToAnchor();
+  window.addEventListener('hashchange', scrollToAnchor);
+}