feat(extensions): Introduce app extensions

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

src/components/markdown-renderer/extensions/base/markdown-renderer-extension.ts

@@ -0,0 +1,39 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type EventEmitter2 from 'eventemitter2'
+import type MarkdownIt from 'markdown-it'
+import type { NodeProcessor } from '../../node-preprocessors/node-processor'
+import type { ComponentReplacer } from '../../replace-components/component-replacer'
+
+/**
+ * Base class for Markdown extensions.
+ */
+export abstract class MarkdownRendererExtension {
+  constructor(protected readonly eventEmitter?: EventEmitter2) {}
+
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public configureMarkdownIt(markdownIt: MarkdownIt): void {
+    return
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public configureMarkdownItPost(markdownIt: MarkdownIt): void {
+    return
+  }
+
+  public buildNodeProcessors(): NodeProcessor[] {
+    return []
+  }
+
+  public buildReplacers(): ComponentReplacer[] {
+    return []
+  }
+
+  public buildTagNameAllowList(): string[] {
+    return []
+  }
+}