Webpack ESM build

[ellthompson]

Webpack can now be called with the environment variable MODULE=true to build .mjs versions of the PCUI library files.

The npm build script now builds umd and esm versions of PCUI.

The readme has been updated to correctly use the .mjs files in it's examples.

Fixes #100

[mvaligursky]

is it common to use environment variable vs passing command line option for this kind of stuff?

[ellthompson]

is it common to use environment variable vs passing command line option for this kind of stuff?

Yeah i'd say so. It's used in the official documentation. It looks like command line variables only cropped up in webpack version 5.

[BruceWheaton]

Just ran into a possibly related build error - do I need to edit my webpack config? I'm using a starter that mimics create-react-app and adds in node/express.
`ERROR in ./node_modules/@playcanvas/pcui/react/dist/pcui-react.mjs 786:16
Module parse failed: Unexpected token (786:16)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
| }
|

attachElement = (nodeElement, containerElement) => {
| if (!nodeElement) return;
| this.element = new this.elementClass({ ...this.props,
@ ./src/components/Example.jsx 2:0-59 14:89-98 14:140-146
@ ./src/App.jsx`