set value of "sideEffects" in package.json to false

[FelixMohr]

What :computer:

Set the value of option "sideEffects" in package.json to false. Same as what ethers.js does: https://github.com/ethers-io/ethers.js/blob/main/package.json

Why :hand:

There are bundled frontend applications that use zksync-ethers. This option will enable tree shaking. Bundles can be organized more efficiently, and in most cases less code will be fetched and parsed by clients, so performance can be improved.

Evidence :camera:

Find more about the option here: https://webpack.js.org/guides/tree-shaking/#clarifying-tree-shaking-and-sideeffects

[petarTxFusion]

Please fix failing jobs. Once you do that we can merge this