Switch to using package.json exports for entrypoints

This allows users to import the module-based version by writing

```
import { BaseRealtime } from 'ably/modules'
```

instead of having to import 'ably/build/modules'.

I followed the guidance in [1] and [2].

[1] https://nodejs.org/api/packages.html#exports
[2] https://webpack.js.org/guides/package-exports/

package.json

@@ -7,14 +7,17 @@
     "url": "https://github.com/ably/ably-js/issues",
     "email": "[email protected]"
   },
-  "main": "./build/ably-node.js",
-  "typings": "./ably.d.ts",
-  "react-native": {
-    "./build/ably-node.js": "./build/ably-reactnative.js"
-  },
-  "browser": {
-    "./build/ably-node.js": "./build/ably.js"
+  "exports": {
+    ".": {
+      "node": "./build/ably-node.js",
+      "react-native": "./build/ably-reactnative.js",
+      "default": "./build/ably.js"
+    },
+    "./modules": {
+      "default": "./build/modules/index.js"
+    }
   },
+  "typings": "./ably.d.ts",
   "files": [
     "build/**",
     "ably.d.ts",