feat(create-rslib): add main/module fields for better compatibility

[Colin3191]

Summary

#1128
Add explicit main and module fields to support:

1. Webpack4 and other legacy bundlers (via main)
2. Modern ES modules consumers (via module)
3. Maintain backward compatibility with exports field

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}