Adding `sideEffects: false` to package.json

Hello! :)

Would it make sense to add `sideEffects: false` to `package.json`? This can enable more tree-shaking, in some cases: [documentation](https://github.com/webpack/webpack/blob/main/examples/side-effects/README.md)

Other popular libraries, such as `axios`, have been adding this flag: https://github.com/axios/axios/pull/5025

I'd be happy to open a PR :)

Start by reviewing package.json and the linked webpack side-effects documentation to understand the proposed metadata. Check whether regexparam has any package-level side effects, then update package.json if appropriate and verify that the published package metadata enables tree-shaking as intended.

Written by the indexing model from the issue text.