chore: updating package.json css path

[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 files field in package.json was not correctly configured to include the CSS file, causing the exports to fail. The previous failed attempts were because I was testing using npm pack instead of yarn pack which render different results.

After this update we should be able to import the style sheet like

import '@metamask/design-tokens/design-tokens.css'

or

import '../../node_modules/@metamask/design-tokens/src/css/design-tokens';

Related issues

Fixes: #608 Attempted previous fixes:

• https://github.com/MetaMask/design-tokens/pull/614
• https://github.com/MetaMask/design-tokens/pull/609

Screenshots/Recordings

Before

https://github.com/MetaMask/design-tokens/assets/8112138/593a791f-096d-45b7-8b82-77505d3597fc

After

https://github.com/MetaMask/design-tokens/assets/8112138/fd314eb6-70aa-41f4-b326-b8ec7e4bdf54

Manual testing steps

• Pull this branch
• Run yarn pack
• Unzip the tar build
• See that the stylesheet is exported in the tar build src/css/design-tokens.css

Pre-merge author checklist

• [x] I’ve followed MetaMask Coding Standards.
• [x] I've clearly explained what problem this PR is solving and how it is solved.
• [x] I've linked related issues
• [x] I've included manual testing steps
• [x] I've included screenshots/recordings if applicable
• [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.
• [x] I’ve properly set the pull request status:
  • [x] 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.

[metamaskbot]

Builds ready [e0df49d]

Storybook: Storybook

[metamaskbot]

Builds ready [7e0a75e]

Storybook: Storybook