create-react-app 创建react应用环境变量（env）配置

参考：https://facebook.github.io/create-react-app/docs/adding-custom-environment-variables

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What other?.env?files can be used?

Note: this feature is?available with?[email?protected]?and higher.

• .env: Default.
• .env.local: Local overrides.?This file is loaded for all environments except test.
• .env.development,?.env.test,?.env.production: Environment-specific settings.
• .env.development.local,?.env.test.local,?.env.production.local: Local overrides of environment-specific settings.

Files on the left have more priority than files on the right:

• npm start:?.env.development.local,?.env.development,?.env.local,?.env
• npm run build:?.env.production.local,?.env.production,?.env
• npm test:?.env.test.local,?.env?(note?.env.local?is missing)