Adds exports."./react" to package.json

Also removes "exports" property from package.react.json in react-hooks
folder that is copied to `react` build folder. This file now only has
`"main": "./cjs/index.js"`, which, for not obvious reason to me, needs
to be present in order for `npx attw` check to not show "Resolution
failed" error for node10 for "ably/react" path.

Resolves #1622

package.json

@@ -17,6 +17,10 @@
     "./modules": {
       "types": "./modules.d.ts",
       "default": "./build/modules/index.js"
+    },
+    "./react": {
+      "require": "./react/cjs/index.js",
+      "import": "./react/mjs/index.js"
     }
   },
   "typings": "./ably.d.ts",

src/platform/react-hooks/res/package.react.json

@@ -1,7 +1,3 @@
 {
-  "main": "./cjs/index.js",
-  "exports": {
-    "require": "./cjs",
-    "import": "./mjs"
-  }
+  "main": "./cjs/index.js"
 }