SPIKE: Investigate CSS modules

[hannalaakso]

What

Based on discussions in #1052, we could do a short spike to explore whether it's feasible for GOV.UK Frontend to support CSS modules by either:

1. Generating an extra [filename].modules.scss file as part of the build process.
2. Renaming our existing scss files to [filename].module.scss - this would be a breaking change.

Why

We know that some teams in the community use Create React App to build services. We'd like to explore whether we can support these teams by making our scss files follow the naming convention.

Tasks

• Investigate options 1 and 2, as well as any other viable options
• Make a recommendation on next steps

[NickColley]

See #1052 (comment)