Fix user-defined markup links targets (go-gitea#29305)

This seeks to fix the bug reported on issue go-gitea#29196. 

Cause: 
ID's with custom characters (- , _ , etc.), were not linking correctly
in the Markdown file when rendered in the browser because the ID in the
respective destinies would be different than the one in anchor, while
for IDs with only letters, the ID would be the same.

Fix:
It was suggested that to fix this bug, it should more or less like
GitHub does it. While in gitea the anchors would be put in HTML like
this:
```
<p dir="auto"><a href="#user-content-_toc152597800" rel="nofollow">Review</a></p>
<p dir="auto"><a href="#user-content-_toc152597802" rel="nofollow">Staging</a></p>
<p dir="auto"><a href="#user-content-_toc152597803" rel="nofollow">Development</a></p>
<p dir="auto"><a href="#user-content-_toc152597828" rel="nofollow">Testing</a></p>
<p dir="auto"><a href="#user-content-_toc152597829" rel="nofollow">Unit-tests</a></p>

```
In GitHub, the same anchor's href properties would be the same without
"user-content-" trailing behind.

So my code made sure to change those anchors, so it would not include
"user-content-" and then add respective Event Listeners so it would
scroll into the supposed places.

Fixes: go-gitea#29196

---------

Co-authored-by: silverwind <me@silverwind.io>

Author: 2 people

web_src/js/markup/anchors.js

@@ -2,6 +2,7 @@ import {svg} from '../svg.js';
 
 const headingSelector = '.markup h1, .markup h2, .markup h3, .markup h4, .markup h5, .markup h6';
 
+// scroll to anchor while respecting the `user-content` prefix that exists on the target
 function scrollToAnchor(hash, initial) {
   // abort if the browser has already scrolled to another anchor during page load
   if (initial && document.querySelector(':target')) return;
@@ -19,6 +20,7 @@ function scrollToAnchor(hash, initial) {
 export function initMarkupAnchors() {
   if (!document.querySelector('.markup')) return;
 
+  // create link icons for markup headings, the resulting link href will remove `user-content-`
   for (const heading of document.querySelectorAll(headingSelector)) {
     const originalId = heading.id.replace(/^user-content-/, '');
     const a = document.createElement('a');
@@ -31,5 +33,18 @@ export function initMarkupAnchors() {
     heading.prepend(a);
   }
 
+  // handle user-defined `name` anchors like `[Link](#link)` linking to `<a name="link"></a>Link`
+  for (const a of document.querySelectorAll('.markup a[href^="#"]')) {
+    const href = a.getAttribute('href');
+    if (!href.startsWith('#user-content-')) continue;
+    const originalId = href.replace(/^#user-content-/, '');
+    a.setAttribute('href', `#${encodeURIComponent(originalId)}`);
+    if (document.getElementsByName(originalId).length !== 1) {
+      a.addEventListener('click', (e) => {
+        scrollToAnchor(e.currentTarget.getAttribute('href'), false);
+      });
+    }
+  }
+
   scrollToAnchor(window.location.hash, true);
 }