fix: refactor mcp servers into separate packages

[reidbarber]

Splits the mcp servers into separate packages. Include a private internal package for shared code.

Before:

npx @react-spectrum/mcp s2
npx @react-spectrum/mcp react-aria

After:

npx @react-spectrum/mcp
npx @react-aria/mcp

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[rspbot]

Build successful! 🎉

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• View the S2 storybook
• View the documentation
• View the S2 docs