Missing styles.css in package.json sideEffects

[Woyken]

Describe the bug (actual behavior)

I'm using webpack with ESM. When importing styles.css it gets treeshaken-out.
And styles are missing completely in output

Possible webpack bug: webpack/webpack#7094

Expected behavior

importing styles.css just works

Reproduction

Link to a minimal repro:
Bug reproduction
https://stackblitz.com/edit/webpack-5-react-starter-zvm3dw?file=src%2Fapp.tsx,package.json

Patched version with "sideEffects": ["styles.css"]
https://stackblitz.com/edit/webpack-5-react-starter-m3a2tr?file=src%2Fapp.tsx,package.json

Steps to reproduce

1. Use webpack bundler with ESM
2. Import iTwinUI styles.css
3. no css in output

Additional information

Possible solution:
Add package.json sideEffects for styles.css file

{
    sideEffects: ["styles.css"]
}

[mayank99]

fixed in 3.1.1, thanks for your PR