[pgAdmin4[Patch][Webpacking]: Enable source maps in pgAdmin4 for development environment

Hi All,

This patch enables source maps in pgAdmin4 for development environment.

On running yarn run bundle It will create source maps(i.e. extract each
module js file so that it is available to debug).

You can search each module(ie. menu.js to debug) file by filename as
follows:

1.

Start pgAdmin4 server or run separately yarn run bundle in pgadmin/web
2.

Open pgAdmin4 in browser(I am using chrome). Click sources panel and
press CMD+P and search for file menu.js. Click and then add breakpoint
to the line where you want to debug codebase. Screenshot attached for
reference.

I have excluded vendor and codemirror bundle from source map as i believe
we don’t debug these files. If someone needs, they can remove vendor.js
from source map plugin and rebuild.

I didn’t add this for production mode yet.

Reference to SourceMapDevToolPlugin
<https://webpack.js.org/plugins/source-map-dev-tool-plugin/#components/sidebar/sidebar.jsx>

Please find attached patch and review.

Thanks,
Surinder
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