fix(css): remove sideEffects package field

[chenxsan]

Summary

When using @docsearch/css with webpack like below:

import '@docsearch/css'

sideEffects: false would cause webpack to prune all the css.

I have to work around with a hack like this:

import styles from '@docsearch/css';
// do something with `styles` to keep it around.

Result

[Haroenv]

this change makes sense according to me, although I haven't yet checked it. The change should happen in a different file though

[Haroenv]

I think this is correct!

[francoischalifour]

Thanks!