Added styles.css to sideEffects

[Woyken]

This fix is intended for webpack users.
When using ESM, webpack will apply it's treeshaking too harshly sometimes.
This applies to imported css files too.
Possible webpack bug: webpack/webpack#7094

Fixes #1737

Changes

Added styles.css to sideEffects in package.json

Testing

"N/A"

Docs

"N/A"

[CLAassistant]

All committers have signed the CLA.

[rohan-kadkol]

After reading the webpack docs about tree-shaking

... and this StackOverflow answer, I believe the change in this PR is alright. But let's wait for another review.

[mayank99]

I've always found sideEffects to be confusing. Using this field in the past, we've had problems with some bundlers including all .css files even if their components are not actually used. But now that we only have a single stylesheet, that should be fine I guess.

I also thought "sideEffects": false would be the correct move, since we do not import it within our components, but apparently not.

My only remaining concern is regarding packages that depend on itwinui-react. If they use a bundler, it might decide to include styles.css even if it's not imported. But packages really should not be using bundlers in the first place, so maybe it's not a big concern.

Lets go with this and see if anybody complains. 👍