Add missing dependency to the website app

[Pavel910]

source-map-explorer in website app is missing as a dependency.

Also, it might be a good idea to add another script to the website app, called build-with-sourcemap, which uses cross-env to set the GENERATE_SOURCEMAP ENV variable, so it works on all OS-es.

Something like this:

"build-with-sourcemap": "cross-env GENERATE_SOURCEMAP=true yarn webiny run build"