feat: adding shadow to tailwind preset

[georgewrmarshall]

Description

This pull request adds new shadow tokens to the design-system-tailwind-preset package. The shadow tokens are derived from the design token CSS variables, and include both color and size definitions. These tokens will generate Tailwind utility classes that ensure consistent shadow styles across components. The shadow configuration has been imported into the index preset for global use.

Related issues

Fixes: https://github.com/MetaMask/metamask-design-system/issues/57

Manual testing steps

1. Run yarn build && yarn to build and install the local version of the tailwind preset in storybook-react
2. Run storybook using yarn storybook
3. See the shadows stories
4. Apply shadow utility classes (shadow-default, shadow-primary, shadow-error, etc.) to verify color.
5. Apply shadow size classes (shadow-xs, shadow-sm, shadow-md, shadow-lg) to check correct shadow sizing.
6. Verify that the shadows match the design tokens for both size and color.

Screenshots/Recordings

Before

N/A (no shadow utilities available)

After

https://github.com/user-attachments/assets/14f0eb19-2a59-4234-89af-f0b8c97949b4

Testing in Portfolio using yarn link

https://github.com/user-attachments/assets/76b6e554-13b4-40b1-83f0-6f540c827fb2

Pre-merge author checklist

• [x] I've followed MetaMask Contributor Docs
• [x] I've completed the PR template to the best of my ability
• [x] I’ve included tests if applicable
• [x] I’ve documented my code using JSDoc format if applicable
• [x] I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• [ ] I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• [ ] I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and/or screenshots.