DefinePlugin cross-compatibility

[jsg2021]

Just updated node modules today, and Storybook started crashing.. the newer webpack exposed where my webpack config used builtin plugins shared to the webpack config of their builder (and presumably their core). My config's plugin instances come from webpack5... However, storybook-core still builds on webpack 4... (its nested under storybook's node_modules)

This change allows the plugin to continue functioning in webpack 4.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[sokra]

We don't provide support for using the webpack 5 DefinePlugin in a webpack 4 build.

[johnnybenson]

This change hurts nothing and makes the software more resilient.

[alexander-akait]

It will not fix nothing, webpack v4 compiler is not the same webpack v5 compiler