Add a field `browser` to `package.json` root

[UNIDY2002]

Fixes https://github.com/cheeriojs/cheerio/issues/4032

Some bundlers (e.g. react-native) requires the field browser at package.json root to find the correct resolution entry points. (Reference: https://metrobundler.dev/docs/configuration/#resolvermainfields)

This PR adds the correct entry point for browsers to package.json.