Update `package.json`

[vladimyr]

• decreasing published tarball size by using files field

• enabling easier CDN consumation through unpkg/jsdelivr fields

• [x] Tests pass

• [ ] Appropriate changes to README are included in PR

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[vladimyr]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[janicklas-ralph]

Thanks for the PR!