refactor: organize app extensions

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

frontend/src/extensions/_base-classes/app-extension.ts

@@ -0,0 +1,43 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import type { FrontendConfig } from '../../api/config/types'
+import type { CheatsheetExtension } from '../../components/editor-page/cheatsheet/cheatsheet-extension'
+import type { Linter } from '../../components/editor-page/editor-pane/linter/linter'
+import type { MarkdownRendererExtension } from '../../components/markdown-renderer/extensions/_base-classes/markdown-renderer-extension'
+import type { RendererType } from '../../components/render-page/window-post-message-communicator/rendering-message'
+import type { CompletionSource } from '@codemirror/autocomplete'
+import type { EventEmitter2 } from 'eventemitter2'
+import type React from 'react'
+import { Fragment } from 'react'
+
+export interface MarkdownRendererExtensionOptions {
+  frontendConfig: FrontendConfig
+  eventEmitter: EventEmitter2
+  rendererType: RendererType
+}
+
+export abstract class AppExtension {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public buildMarkdownRendererExtensions(options: MarkdownRendererExtensionOptions): MarkdownRendererExtension[] {
+    return []
+  }
+
+  public buildCodeMirrorLinter(): Linter[] {
+    return []
+  }
+
+  public buildEditorExtensionComponent(): React.FC {
+    return Fragment
+  }
+
+  public buildCheatsheetExtensions(): CheatsheetExtension[] {
+    return []
+  }
+
+  public buildAutocompletion(): CompletionSource[] {
+    return []
+  }
+}