Adding brand colors to CSSinJS exports

[georgewrmarshall]

Description

This PR adds the brandColors object to the exports of the design tokens package. It also includes unit tests to ensure the brandColors object matches the Figma design token values.

1. Added brandColors object to the design tokens exports.
2. Created unit tests for brandColors to ensure it matches the Figma design tokens values.

Fixes: #581

Usage

import { brandColors } from '@metamask/design-tokens';

<Icon name={IconName.Star} color={brandColors.yellow.yellow500} />

Manual testing steps

1. Pull this branch
2. Run the unit tests and ensure all tests pass.
3. Ensure brandColors is exported from project index

[metamaskbot]

Builds ready [5fab2d5]

• storybook: Storybook
• all artifacts