Adding an API for inter-iframe communication

Adds a first version of an API to communicate between an iFrame opened by WorkAdventure and WorkAdventure itself.
The first API method is a method allowing to add messages in the chat, from the iFrame.

Comes with a test file.

front/src/iframe_api.ts

@@ -0,0 +1,21 @@
+interface WorkAdventureApi {
+    sendChatMessage(message: string, author: string): void;
+}
+
+declare var WA: WorkAdventureApi;
+
+window.WA = {
+    /**
+     * Sends a message in the chat.
+     * Only the local user will receive this message.
+     */
+    sendChatMessage(message: string, author: string) {
+        window.parent.postMessage({
+            'type': 'chat',
+            'data': {
+                'message': message,
+                'author': author
+            }
+        }, '*');
+    }
+}