webpack configuration can also use the .cjs extension

[shazron]

Issue in /main/src/pages/guides/app_builder_guides/configuration/webpack-configuration.md

.cjs is also allowed.
See https://github.com/adobe/aio-lib-runtime/blob/2526e9b64a2de6901bf99bb910d69a06ed22e716/src/build-actions.js#L49

This is important if an App Builder project's package.json has
"type": "module" which signifies their code is all ESM.

Action item:

• update all references to the webpack config file name extension ending with .js to also allow .cjs