feat(extensions): Introduce app extensions

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

src/components/markdown-renderer/extensions/sanitizer/dom-purifier-node-preprocessor.ts

@@ -0,0 +1,29 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Document } from 'domhandler'
+import render from 'dom-serializer'
+import DOMPurify from 'dompurify'
+import { parseDocument } from 'htmlparser2'
+import { NodeProcessor } from '../../node-preprocessors/node-processor'
+
+/**
+ * Sanitizes the given {@link Document document}.
+ *
+ * @see https://cure53.de/purify
+ */
+export class SanitizerNodePreprocessor extends NodeProcessor {
+  constructor(private tagNameWhiteList: string[]) {
+    super()
+  }
+
+  process(nodes: Document): Document {
+    const sanitizedHtml = DOMPurify.sanitize(render(nodes), {
+      ADD_TAGS: this.tagNameWhiteList
+    })
+    return parseDocument(sanitizedHtml)
+  }
+}