Use api.env in babel-preset-react-app

Created by: vikr01

See docs for api.env. There are various ways of setting the envName for babel, and this makes sure that all are taken into account.

If accessing process.env.BABEL_ENV and process.env.NODE_ENV directly is still wanted, we should use api.cache.using.

I also noticed @babel/core was in dependencies when it shouldn't be (since users will install @babel/core to use the preset with), this usually happens after running babel-upgrade. It should go in peerDependencies.

packages/babel-preset-react-app/package.json

@@ -16,8 +16,10 @@
     "prod.js",
     "test.js"
   ],
+  "peerDependencies": {
+    "@babel/core": "7.2.2"
+  },
   "dependencies": {
-    "@babel/core": "7.2.2",
     "@babel/plugin-proposal-class-properties": "7.3.0",
     "@babel/plugin-proposal-decorators": "7.3.0",
     "@babel/plugin-proposal-object-rest-spread": "7.3.2",