fix: Allow stylesheets to be bundled in storybook production build

[frankieyan]

Short description

Fixes Storybook's production build so that stylesheets are included again.

This one has actually been lingering around since v6.0.0 - because we declared our package as side-effect-free (and later having declared CSS files being the only side-effects), Webpack drops our *.less imports from the production build as we simply import them, rather than passing their reference around as we do with CSS modules.

See https://webpack.js.org/configuration/module/#rulesideeffects and https://webpack.js.org/guides/tree-shaking for more details.

Since we don't expose any *.less files in the tsdx bundles, we should leave package.json as is and only declare this with Storybook's Webpack config.

Test plan

• Open up the /docs folder in a browser (or use something like VSCode's Live Server. Verify that styles are being loaded.

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Updated all static build artifacts (npm run build-all)

Versioning

No need for a release here

[henningmu]

Great going for the root cause – works like a charm 👏