Add pureBlackDarkTheme to @metamask/design-tokens

[georgewrmarshall]

Description

Add shared types for pure-black provider/context state so React and React Native consume the same contract.

Acceptance Criteria

• Shared exports exist for PureBlackContextValue and PureBlackProviderProps
• React and React Native packages reuse the shared contract
• Package builds cleanly

Constraint

• isPureBlack=false uses the existing grey dark theme
• isPureBlack=true uses pureBlackDarkTheme

References

• Jira epic: TMCU-622