Improve heft-webpack-basic-tutorial to illustrate .scss support

[octogonz]

• Upgrade to Webpack 5
• Enable webpack.config.js options for .scss support
• Improve webpack.config.js docs
• Add examples of .scss usage

Over time I'm hoping for this project to accumulate a reasonable set of "best practices" for a real world web application. Although it's a "tutorial", I'd like to to aim for this sample to be:

• full featured
• well documented
• realistic

That seems more useful to people than trying to make it simplistic.

These same settings will be integrated into the @rushstack/heft-web-rig setup. Then we'll add a heft-webpack-rig-tutorial, and heft-webpack-basic-tutorial will be the same thing but with all the rig settings inlined.

CC @iclanton @halfnibble