feat(extensions): Introduce app extensions

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/components/markdown-renderer/extensions/headline-anchors-markdown-extension.ts

@@ -0,0 +1,25 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { MarkdownRendererExtension } from './base/markdown-renderer-extension'
+import type MarkdownIt from 'markdown-it'
+import anchor from 'markdown-it-anchor'
+
+/**
+ * Adds headline anchors to the markdown rendering.
+ */
+export class HeadlineAnchorsMarkdownExtension extends MarkdownRendererExtension {
+  public configureMarkdownIt(markdownIt: MarkdownIt): void {
+    anchor(markdownIt, {
+      permalink: anchor.permalink.ariaHidden({
+        symbol: '<i class="fa fa-link"></i>',
+        class: 'heading-anchor text-dark',
+        renderHref: (slug: string): string => `#${slug}`,
+        placement: 'before'
+      })
+    })
+  }
+}