[landing] added sideEffects to solve node_module CSS (rainbowkit) not being added in production

Summary: RainbowKit's CSS doesn't get bundled in production, because the way they're exposing it is with a sideEffect. Webpack's side effects are basically an infinite black hole, as denoted by this issue that's been open for 4.5 years. more context/explanation on this stack overflow answer

Test Plan: run yarn prod in landing with this diff and RainbowKit experience found on localhost/commlanding/siwe now has more than 0 styles

Reviewers: atul, varun, abosh, ashoat

Reviewed By: ashoat

Subscribers: tomek, abosh, varun, ashoat, atul

Differential Revision: https://phab.comm.dev/D5224