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/in-memory-connection-message.transporter.ts

@@ -0,0 +1,52 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import {
+  ConnectionState,
+  MessageTransporter
+} from '../message-transporters/message-transporter.js'
+import { Message, MessageType } from '../message-transporters/message.js'
+
+/**
+ * Message transporter for testing purposes that redirects message to another in memory connection message transporter instance.
+ */
+export class InMemoryConnectionMessageTransporter extends MessageTransporter {
+  private otherSide: InMemoryConnectionMessageTransporter | undefined
+
+  constructor(private name: string) {
+    super()
+  }
+
+  public connect(other: InMemoryConnectionMessageTransporter): void {
+    this.otherSide = other
+    other.otherSide = this
+    this.onConnected()
+    other.onConnected()
+  }
+
+  public disconnect(): void {
+    this.onDisconnecting()
+
+    if (this.otherSide) {
+      this.otherSide.onDisconnecting()
+      this.otherSide.otherSide = undefined
+      this.otherSide = undefined
+    }
+  }
+
+  sendMessage(content: Message<MessageType>): void {
+    if (this.otherSide === undefined) {
+      throw new Error('Disconnected')
+    }
+    console.debug(`${this.name}`, 'Sending', content)
+    this.otherSide?.receiveMessage(content)
+  }
+
+  getConnectionState(): ConnectionState {
+    return this.otherSide !== undefined
+      ? ConnectionState.CONNECTED
+      : ConnectionState.DISCONNECT
+  }
+}