Introduce Markdown extensions (#1614)

* Introduce markdown extensions

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

src/components/markdown-renderer/markdown-extension/gist/gist-markdown-extension.ts

@@ -0,0 +1,34 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { MarkdownExtension } from '../markdown-extension'
+import markdownItRegex from 'markdown-it-regex'
+import type MarkdownIt from 'markdown-it'
+import type { ComponentReplacer } from '../../replace-components/component-replacer'
+import { CustomTagWithIdComponentReplacer } from '../../replace-components/custom-tag-with-id-component-replacer'
+import { replaceGistLink } from './replace-gist-link'
+import { replaceLegacyGistShortCode } from './replace-legacy-gist-short-code'
+import { GistFrame } from './gist-frame'
+
+/**
+ * Adds support for embeddings of GitHub Gists by detecting gist links and the legacy gist shortcode.
+ */
+export class GistMarkdownExtension extends MarkdownExtension {
+  public static readonly tagName = 'app-gist'
+
+  public configureMarkdownIt(markdownIt: MarkdownIt): void {
+    markdownItRegex(markdownIt, replaceGistLink)
+    markdownItRegex(markdownIt, replaceLegacyGistShortCode)
+  }
+
+  public buildReplacers(): ComponentReplacer[] {
+    return [new CustomTagWithIdComponentReplacer(GistFrame, GistMarkdownExtension.tagName)]
+  }
+
+  public buildTagNameWhitelist(): string[] {
+    return [GistMarkdownExtension.tagName]
+  }
+}