[pgAdmin4][Patch]: Add command under scripts to run webpack bundle in watch mode

Hi

Currently, whenever we are making a change in JS file, we have to re-run
bundle script again to generate all bundles again, which takes time and it
is pain for developers to watch it till finished.

So Webpack `watch` option lets the Webapck watch for changes in the file
and compiles only files which are changed after the initial build.

Also added an option `--progress` which updates on terminal about packages
being built and percentage completed.

So, you can run bundle in watch mode in terminal:

``` $ yarn run bundle:watch ```

Reference [link](
https://webpack.js.org/configuration/watch/#components/sidebar/sidebar.jsx)

Please review the patch

Thanks,
Surinder Kumar