bug: fix css exports

[georgewrmarshall]

Description

This PR addresses an issue where the CSS stylesheet exports were not functioning correctly due to a misconfiguration in the package.json file. Specifically, the exports field in package.json was not correctly configured to include the CSS file, causing the exports to fail.

Related issues

Fixes: #608
Attempted previous fix: #609

Screenshots/Recordings

This PR does not include any visual changes, so screenshots are not applicable.

Before

before.mov

After

after.mov

Manual testing steps

• Pull this branch
• Run npm pack
• Unzip the tar build
• Renaing to "design-tokens"
• Replace the design tokens folder in node_modules with new "design-tokens" folder in the extension
• Run yarn start in the extension

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.
• I’ve properly set the pull request status:
  • In case it's not yet "ready for review", I've set it to "draft".
  • In case it's "ready for review", I've changed it from "draft" to "non-draft".

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.