Introduce Markdown extensions (#1614)

* Introduce markdown extensions

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

src/components/markdown-renderer/markdown-extension/table-of-contents-markdown-extension.ts

@@ -0,0 +1,32 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { MarkdownExtension } from './markdown-extension'
+import type MarkdownIt from 'markdown-it'
+import type { TocAst } from 'markdown-it-toc-done-right'
+import toc from 'markdown-it-toc-done-right'
+import { tocSlugify } from '../../editor-page/table-of-contents/toc-slugify'
+
+export class TableOfContentsMarkdownExtension extends MarkdownExtension {
+  constructor(private onTocChange?: (ast: TocAst) => void) {
+    super()
+  }
+
+  public configureMarkdownIt(markdownIt: MarkdownIt): void {
+    if (!this.onTocChange) {
+      return
+    }
+    toc(markdownIt, {
+      placeholder: '(\\[TOC\\]|\\[toc\\])',
+      listType: 'ul',
+      level: [1, 2, 3],
+      callback: (code: string, ast: TocAst): void => {
+        this.onTocChange?.(ast)
+      },
+      slugify: tocSlugify
+    })
+  }
+}