Add a package sideEffects field

[jaydenseric]

This PR adds a package sideEffects field with a false value. This allows tree-shaking and dead-code-elimination to work much better, eliminating imports of prop-types if all of the imports end up being unused.

• esbuild docs: https://esbuild.github.io/api/#manual-tree-shaking-annotations
• webpack docs: https://webpack.js.org/guides/tree-shaking

This change could significantly improve the bundle size of projects that nest propTypes under a conditional so they will be eliminated from a production bundle, for example:

https://github.com/jaydenseric/device-agnostic-ui/blob/89e5ba0b8c0ee5a8fb38f4ed74e7f19a6b4970d7/src/public/components/Button.js#L25-L32

if (typeof process === 'object' && process.env.NODE_ENV !== 'production') {
  Button.displayName = 'Button';
  Button.propTypes = {
    disabled: PropTypes.bool,
    className: PropTypes.string,
    children: propTypeChildren.isRequired,
  };
}

Currently, because the prop-types package doesn't have a "sideEffects": false field, for a production bundle esbuild eliminates the Button.propTypes = { declaration but it leaves the const PropTypes = require('prop-types');.

[ljharb]

it's a non-standard field, but since it'll never be a standard one, it won't hurt

[jaydenseric]

@ljharb can we please publish this change? Been waiting 6 months 😅

[ljharb]

v15.8.0 is released.