Upgrade to webpack@4

In an effort to provide security releases, we should investigate upgrading the bundling of this library to use webpack@4. This will remove our dependency on node-gyp downstream, as the latest versions no longer depend on it. This may also require us to rev the node version to something higher than >8 (more modern).

Steps to accomplish this upgrade:

• Increase package versions
• Remove DefinePlugin and UglifyJsPlugin, use compress options instead
• Set mode=production in webpack config
• Ensure config for test is appropriate (possibly mode=development|none)