refactor: reimplement realtime-communication

This commit refactors a lot of things that are not easy to separate.
It replaces the binary protocol of y-protocols with json.
It introduces event based message processing.
It implements our own code mirror plugins for synchronisation of content and remote cursors

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

commons/src/y-doc-sync/realtime-doc.test.ts

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { RealtimeDoc } from './realtime-doc.js'
+import { describe, expect, it } from '@jest/globals'
+
+describe('websocket-doc', () => {
+  it('saves the initial content', () => {
+    const textContent = 'textContent'
+    const websocketDoc = new RealtimeDoc(textContent)
+
+    expect(websocketDoc.getCurrentContent()).toBe(textContent)
+  })
+})