Add sideEffects field to package.json

[ggetz]

As mentioned in #9212 (comment), the sideEffects field in package.json is necessary to enable tree shaking in conservative build tools such as Webpack. CesiumJS has generally maintained a fairly agnostic package.json file, but I think it's worth it to include such a field in this case since it's a dramatic optimization benefit for a popular build tool. While specific to bundlers like Webpack, it's common that libraries, such as three.js and BabylonJS, define it.

This can be tested with a fairly minimal Webpack config, eg. https://github.com/CesiumGS/cesium-webpack-example. Without sideEffects defined, the default app is 17.2MB bundled. With sideEffects, the default app drops to 14.2MB bundled. If you have an even smaller example, say and app that only imports Cartesian3, it can drop to ~100KB.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[sanjeetsuhag]

Using the default application in cesium-webpack-example, I compared the build sizes:

Build Mode	main	side-effects
development	23.5 MB	20.8 MB
production	19.4 MB	17.6 MB

I modified index.js to only include Cartesian3 and then I compared the build sizes:

Build Mode	main	side-effects
development	23.4 MB	7.6 MB
production	19.3 MB	7.6 MB

Note: I had to add the following property to resolve in webpack.config.js to get things to work on the main branch of cesium-webpack-example:

fallback: {
    http: false,
    https: false,
    zlib: false,
}

[sanjeetsuhag]

Thanks @ggetz