chore: added index.js for structures (#91)

- Allows for a cleaner integration (imo) in Client.js

src/structures/index.js

@@ -0,0 +1,13 @@
+module.exports = {
+    Base: require('./Base'),
+    BusinessContact: require('./BusinessContact'),
+    Chat: require('./Chat'),
+    ClientInfo: require('./ClientInfo'),
+    Contact: require('./Contact'),
+    GroupChat: require('./GroupChat'),
+    Location: require('./Location'),
+    Message: require('./Message'),
+    MessageMedia: require('./MessageMedia'),
+    PrivateChat: require('./PrivateChat'),
+    PrivateContact: require('./PrivateContact'),
+};