Add `scss/customProperties` format in style-dictionary config

Update design token imports to use src/tokens/build directory

feat(DesignTokens): Generate SCSS output with CSS custom property values

See merge request gitlab-org/gitlab-ui!4200



Merged-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Approved-by: Thomas Hutterer <thutterer@gitlab.com>
Approved-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>

refactor: Update design token imports to use src/tokens/build directory

See merge request gitlab-org/gitlab-ui!4202



Merged-by: Scott de Jonge <sdejonge@gitlab.com>
Approved-by: Rahul Chanila <rchanila@gitlab.com>
Approved-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Reviewed-by: Rahul Chanila <rchanila@gitlab.com>

# [80.11.0](gitlab-org/gitlab-ui@v80.10.0...v80.11.0) (2024-05-17)

### Features

* **DesignTokens:** Generate SCSS output with CSS custom property values ([3b92239a](gitlab-org/gitlab-ui@3b92239a))