feat: adding avatar token to design system react

[georgewrmarshall]

Has a dependency on AvatarBase #390

Description

This PR adds the new AvatarToken component to design system react

Key features:

• Displays token logos with consistent sizing and styling
• Provides fallback text display when images are unavailable
• Supports multiple size variants
• Built on top of the AvatarBase component
• Fully tested and documented

Related issues

Fixes: #364

Manual testing steps

1. Run Storybook (yarn storybook)
2. Navigate to React Components/AvatarToken
3. Test different component variations:
   • With and without images
   • Different sizes
   • Fallback text behavior
   • Custom className application
4. Verify responsive behavior and image handling
5. Check accessibility features (alt text, etc.)

Screenshots/Recordings

Before

N/A (New component)

After

after720.mov

Pre-merge author checklist

• I've followed MetaMask Contributor Docs
• I've completed the PR template to the best of my ability
• I've included comprehensive tests for the AvatarToken component
• I've documented the code using JSDoc format
• I've applied the right labels on the PR
• I've ensured the component meets accessibility standards

Pre-merge reviewer checklist

• I've manually tested the PR in Storybook
• I confirm that this PR addresses all acceptance criteria for the AvatarToken component
• I've verified the component works correctly with and without images
• I've checked that the component integrates properly with the existing design system

[georgewrmarshall]

Adding missing AvatarBase exports to root index