Should fix #37

Google Colab and VSCode rely on loading the bundled extension (dist/index.js) from a CDN. I believe Colab uses unpkg, but others may use jsdelivr, etc.

CDNs generally defer to the main field of the package.json (since this is how Node's resolution algorithm works), but there are unofficial fields like unpkg, jsdelivr, module, browser, etc. It would be simple enough to make the main field dist/index.js to support most CDNs, but then that breaks jupyterlab extension build (which requires the main to be src/index.js). Fix one leak and another opens 🙃

This PR uses publishConfig – an formal field used by npm (and other package managers) to override properties in the package.json at publish-time (e.g., when running npm publish). This means main remains src/index.js during development and when running all build commands, and then is changed to dist/index.js only when publish to npm.

In my experience, this is the easiest fix - especially since the source code (js/src/index.js) isn't useable in any JS runtime nor would be used by others with a bundler since it's so specific for the Jupyter-like clients.