feat(extensions): Introduce app extensions

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

src/extensions/base/app-extension.ts

@@ -0,0 +1,26 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Linter } from '../../components/editor-page/editor-pane/linter/linter'
+import type { MarkdownRendererExtension } from '../../components/markdown-renderer/extensions/base/markdown-renderer-extension'
+import type React from 'react'
+import { Fragment } from 'react'
+import type EventEmitter2 from 'eventemitter2'
+
+export abstract class AppExtension {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public buildMarkdownRendererExtensions(eventEmitter?: EventEmitter2): MarkdownRendererExtension[] {
+    return []
+  }
+
+  public buildCodeMirrorLinter(): Linter[] {
+    return []
+  }
+
+  public buildEditorExtensionComponent(): React.FC {
+    return Fragment
+  }
+}