[@react-spectrum/icon] peer dependencies

[joelrbrandt]

• add @adobe/react-spectrum as a peer dependency

• mark both @adobe/react-spectrum and @react-spectrum/provider as optional peer dependencies

Closes #1713

✅ Pull Request Checklist:

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

📝 Test Instructions:

1. Use a package manager that is strict about peer dependencies (e.g. PNPM)
2. Create a project that has @adobe/react-spectrum as a dependency
3. Add @react-spectrum/icon as another dependency
4. Confirm there are no peer dependency errors

🧢 Your Project:

Adobe / Adobe Research / internal project