chore: Update Storybook bundler to webpack 5

[scinos]

Changes proposed in this Pull Request

• Use webpack5 builder in Storybook (see https://gist.github.com/shilman/8856ea1786dcd247139b47b270912324#upgrading-from-webpack-4)
• Refactor shared config to accept plugins
• Use ProvidePlugin to define process in @automattic/search storybook

Testing instructions

Run these commands and verify the load storybook with stories and there are no warnings in the browser console:

• yarn components:storybook:start
• yarn search:storybook:start
• yarn composite-checkout:storybook:start

[github-actions]

Link to Calypso live: https://calypso.live?image=registry.a8c.com/calypso/app:build-14977
Link to Jetpack Cloud live: https://calypso.live?image=registry.a8c.com/calypso/app:build-14977&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~1437 bytes added 📈 [gzipped])

name     parsed_size           gzip_size
media        +1030 B  (+0.1%)    +1437 B  (+0.3%)
devdocs      +1030 B  (+0.6%)    +1437 B  (+3.3%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~1437 bytes added 📈 [gzipped])

name                                               parsed_size           gzip_size
async-load-calypso-post-editor-media-modal             +1030 B  (+0.1%)    +1437 B  (+0.4%)
async-load-calypso-post-editor-editor-media-modal      +1030 B  (+0.1%)    +1437 B  (+0.3%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[scinos]

I merged the two Storybook jobs into one, and added composite-checkout.

Also, I dropped NODE_ENV="production". It was causing problems, because Storybook sets it to development internally, and webpack's DefinePlugin was failing with storybookjs/storybook#14257 (comment)

[noahtallen]

Makes sense

[noahtallen]

Tests well!