add "browser" entry to package.json

[redbrain]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update [ ] Bug fix [X] New feature [ ] Other

What changes did you make? (Give an overview)

• In package.json, set "browser" to "simplepeer.min.js". This allows CDNs such as JSDelivr to serve the prebuilt file by default.

Which issue (if any) does this pull request address? N/A

Is there anything you'd like reviewers to focus on? No