USWDS-Sandbox - Dependencies: Add stylelint & prettier

Add style linting and formatting for consistent code style and formatting.

### Definition of done

>**Note**
>Config has changed with stylelint 15+
>https://stylelint.io/migration-guide/to-15

- [ ] NPM Packages
  - [ ] @18f/identity-stylelint-config
  - [ ] prettier
  - [ ] stylelint
- [ ] An NPM script for checking rules.
- [ ] An NPM script for fixing rules.

