Re-add main and browser fields to package.json

Those fields were removed in de5ddfa
and replaced with `exports` field we currently use. However, removal of
root level entry fields from `package.json` have caused issues with
importing `ably` in some setups:
- React Native wasn't able to resolve `ably` at all, due to `exports`
  field resolution is not enabled by default yet [1] (we fixed this by
  re-adding root level `react-native` field in #1713)
- older bundler versions do not support `exports` field, for example,
  webpack has added support for `exports` only in v5.
- TypeScript added support for resolving `exports` field only in its 4.7
  release [2]

This commit re-adds root level entry fields so those setups will still
be able to resolve `ably` package.

Resolves #1751

[1] https://reactnative.dev/blog/2023/06/21/package-exports-support
[2] https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-7.html#packagejson-exports-imports-and-self-referencing

package.json

@@ -7,7 +7,10 @@
     "url": "https://github.com/ably/ably-js/issues",
     "email": "[email protected]"
   },
+  "main": "./build/ably-node.js",
+  "browser": "./build/ably.js",
   "react-native": "./build/ably-reactnative.js",
+  "typings": "./ably.d.ts",
   "exports": {
     ".": {
       "types": "./ably.d.ts",
@@ -24,7 +27,6 @@
       "import": "./react/mjs/index.js"
     }
   },
-  "typings": "./ably.d.ts",
   "files": [
     "build/**",
     "ably.d.ts",