Add more example-lib tests; use sass modern API

[ecraig12345]

Example the just-example-lib just.config.ts and other files to provide more realistic E2E tests of the tasks from just-scripts.

Fix some issues this revealed:

• sassTask: use the sass modern compile() API when available
• styleOverlay: fix how options are passed to postcss-loader
• Webpack tasks: check for all webpack config extensions, and support default exports from the config file