Add shared pure-black provider contract to @metamask/design-system-shared

[georgewrmarshall]

Description

Extend the React Native ThemeProvider to accept isPureBlack?: boolean, switch dark-mode token generation to pureBlackDarkTheme when enabled, and export usePureBlack().

Acceptance Criteria

• ThemeProvider accepts isPureBlack
• theme === Theme.Dark && isPureBlack uses pureBlackDarkTheme
• usePureBlack() is exported and returns correct state
• Existing dark theme behavior is unchanged when isPureBlack is false

Constraint

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

References

• Jira epic: TMCU-622