Use webpack IgnorePlugin to exclude 'react-is' from production build

[dav-is]

react-is isn't used in production, so we shouldn't bundle it.

Note: most of those plugins are using the dev variable, but in case someone runs NODE_ENV=development next build, they would need a copy of react-is because the conditional use of react-is checks NODE_ENV — not whether or not HMR is being used (what what the dev variable is based on).

[timneutkens]

process.env.NODE_ENV is always set to production when !dev, because webpack mode: 'production' sets it.