Creating a dedicated package for iframe API typings

If users are willing to use Typescript to develop scripts for maps, they will need typings for the `WA` object.
This commit creates a new package (@workadventure/iframe-api-typings) that contains only the `iframe_api.d.ts` file.

The file is generated from the build of the front and isolated in this package.
This is necessary because the iframe_api.js file is supposed to always be loaded from WorkAdventure directly (and there is no @workadventure/iframe-api package and there will never be one)

front/packages/iframe-api-typings/.gitignore

@@ -0,0 +1 @@
+iframe_api.d.ts

front/packages/iframe-api-typings/README.md

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+<h1 align="center">WorkAdventure - IFrame API typings for Typescript</h1>
+
+<p align="center">This package contains Typescript typings for <a href="https://workadventu.re/map-building/scripting">WorkAdventure's map scripting API</a></p>
+
+<hr/>
+
+[WorkAdventure](https://workadventu.re) comes with a scripting API. Using this API, you can add some intelligence to your map.
+You use this API by loading an external script directly from WorkAdventure (at https://play.workadventu.re/iframe_api.js), or this script is loaded
+for you if you are using the "script" property of a map.
+
+This project contains Typescript typings for the `WA` object provided by this script.
+
+## Usage
+
+This package is only useful if you are using Typescript to script your WorkAdventure maps.
+
+## Download & Installation
+
+```shell
+$ npm install @workadventure/iframe-api-typings
+```
+
+or
+
+```shell
+$ yarn add @workadventure/iframe-api-typings
+```

front/packages/iframe-api-typings/iframe_api.js

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+// This file is voluntarily empty.

front/packages/iframe-api-typings/package.json

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+{
+  "name": "@workadventure/iframe-api-typings",
+  "version": "1.2.0",
+  "description": "Typescript typings for WorkAdventure iFrame API",
+  "main": "iframe_api.js",
+  "types": "iframe_api.d.ts",
+  "repository": "https://github.com/thecodingmachine/workadventure/",
+  "author": "David Négrier <d.negrier@thecodingmachine.com>",
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  }
+}

front/tsconfig.json

@@ -5,6 +5,7 @@
     "moduleResolution": "node",
     "module": "CommonJS",
     "target": "ES2015",
+    "declaration": true,
     "downlevelIteration": true,
     "jsx": "react",
     "allowJs": true,