Include conditional exports in package.json

Currently, we have main and module in the package.json which specify the entry point for cjs and esm. ref

exports is supported by Node.js 12+ ref

Note that exports will take precedence over the main field: https://nodejs.org/api/packages.html#main

The exports field can produce extra extensibility, for example, tools like webpack will know some specific fields. ref

BTW, this is how azure sdk does: https://github.com/Azure/azure-sdk-for-js/blob/main/sdk/core/core-auth/package.json
I prefer to follow azure sdk.