Added SASS support to Vite app and restructured styles imports

[WolfyUK]

Context

AB#247410 AB#247409

Change proposed in this pull request

Replaced the single index.css with multiple styles.scss files alongside each dependent component in Vite folder convention. Re-used govuk-frontend variables and mixins where available, rather than duplicate the effort.

Guidance to review

Source maps could not be generated from the SASS at this time. Otherwise, validate in the Vite dev server and/or a prod build-and-copy to Web from front-end after first updating npm dependencies.

Checklist (add/remove as appropriate)

• Work items have been linked (use AB#)
• Your code builds clean without any errors or warnings
• You have run all unit/integration tests and they pass
• Your branch has been rebased onto main
• You have tested by running locally
• You have reviewed with UX/Design