Add basic multiplayer

shared/src/commands/broadcast-commands.ts

@@ -0,0 +1,7 @@
+import { CommandReceiver } from './command-receiver';
+import { CommandGenerator } from './command-generator';
+
+export const broadcastCommands = (
+  commandGenerators: Array<CommandGenerator>,
+  commandReceivers: Array<CommandReceiver>
+) => commandReceivers.forEach((r) => commandGenerators.forEach((g) => g.subscribe(r)));