docs: replace BABEL_ENV with NODE_ENV (#1561)

According to the [babel documentation](https://babeljs.io/docs/en/options#envname),
`NODE_ENV` can also be recognized when loading configurations, and
`NODE_ENV` has better interoperability in the ecosystem.

Here's a use case that `NODE_ENV` works well while `BABEL_ENV` might
cause confusion: vuejs/vue-cli#5525 (comment)

Author: haoqunjiang

docs/guides/testing-single-file-components-with-karma.md

@@ -136,13 +136,13 @@ Install `karma-coverage`, `babel-plugin-istanbul`, and `cross-env`:
 npm install --save-dev karma-coverage cross-env
 ```
 
-We're going to use `cross-env` to set a `BABEL_ENV` environment variable. This way we can use `babel-plugin-istanbul` when we're compiling for our tests—we don't want to include `babel-plugin-istanbul` when we compile our production code:
+We're going to use `cross-env` to set a `NODE_ENV` environment variable. This way we can use `babel-plugin-istanbul` when we're compiling for our tests—we don't want to include `babel-plugin-istanbul` when we compile our production code:
 
 ```
 npm install --save-dev babel-plugin-istanbul
 ```
 
-Update your `.babelrc` file to use `babel-plugin-istanbul` when `BABEL_ENV` is set to test:
+Update your `.babelrc` file to use `babel-plugin-istanbul` when `NODE_ENV` is set to test:
 
 ```json
 {
@@ -174,13 +174,13 @@ module.exports = function(config) {
 }
 ```
 
-And update the `test` script to set the `BABEL_ENV`:
+And update the `test` script to set the `NODE_ENV`:
 
 ```json
 // package.json
 {
   "scripts": {
-    "test": "cross-env BABEL_ENV=test karma start --single-run"
+    "test": "cross-env NODE_ENV=test karma start --single-run"
   }
 }
 ```