Core: Support manager build with webpack5

[shilman]

Issue: #14044

NOTE: I released 6.3.0-alpha.37 / 38 / 39 off this feature branch for testing purposes!

What I did

• Extract manager-webpack4 from core-server
• Add manager-webpack5 that uses webpack5-compatible plugins
• Other minor refactors
• Fix warning ("expected 4, received 4")
• import * as webpackReal for addons Presets: Expand webpackInstance to include entire namespace #15016
• Add warning when you don't have manager-webpack5, link to MIGRATION.md

Self-merging @tmeasday @ndelangen @wKich @yannbf

How to test

• CI passes (ish)
• Released on alpha & tested in sample app

yarn add @storybook/builder-webpack5 @storybook/manager-webpack5 webpack --dev

Then update .storybook/main.js:

module.exports = {
  core: {
    builder: 'webpack5'
  }
}

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