Add a PUSHER_HTTP_PORT configuration variable.

Some deployments might not be based on docker, and not on separate
hosts. In such cases, it would be useful to be able to configure the
port pusher listens on.

pusher/server.ts

@@ -1,3 +1,4 @@
 // lib/server.ts
 import App from "./src/App";
-App.listen(8080, () => console.log(`WorkAdventure starting on port 8080!`))
+import { PUSHER_HTTP_PORT } from "./src/Enum/EnvironmentVariable";
+App.listen(PUSHER_HTTP_PORT, () => console.log(`WorkAdventure starting on port ${PUSHER_HTTP_PORT}!`))