Create Figma style-dictionary format

What does this MR do?

This MR implements a Figma style-dictionary format to be used in !5547 (merged) to sync design tokens with the Figma API. It's not currently used by any working tooling and has no risk.

Changes to token values to support Figma should be validated in *.css/*.scss output, but should be safe.

Replace previous directory filtering and file concatenation approach with native style-dictionary format to generate Figma structured design token JSON.

The Figma format:

1. Flattens design tokens e.g. font.size.500 becomes font-size-500
2. Resolves types and values to those supported by Figma
3. Filters out $deprecated: true design tokens
4. Exports as:
   1. src/tokens/figma/constants.json
   2. src/tokens/figma/constants.dark.json (largly redundant as values are the same but helpful to reduce complexity in style-dictionary mode automation)
   3. src/tokens/figma/mode.json
   4. src/tokens/figma/mode.dark.json

The generated output includes the token.original JSON design token content which aligns with Design Token Community Group specification.

Using style-dictionary configuration ensures default and dark mode values are automatically selected with each preprocessor so that src/tokens/figma/mode.json contains the default value and src/tokens/figma/mode.dark.json contains the dark value for each design token.

Screenshots or screen recordings

Integrations

n/a

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Components

• GitLab UI's contributing guidelines.
• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.