Rename and export defaultSlot and remove slotCallbackSymbol from RAC

[alexasselin008]

Closes #5671

• Renamed defaultSlot to DEFAULT_SLOT to follow the naming convention used by other exported symbols.
• Exported DEFAULT_SLOT from react-aria-components.
• Removed the slotCallback symbol as suggested by @devongovett in this comment: Export defaultSlot in RAC or a way to workaround required slot errors #5671 (comment).
• Added documentation in the Advanced Customization part of the documentation, as suggested by @LFDanLu in this comment: Export defaultSlot in RAC or a way to workaround required slot errors #5671 (comment).

Of course, the hardest part of this PR was the documentation, so feel free to suggest any changes, and I'll happily make them.

✅ 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:

[snowystinger]

GET_BUILD

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[LFDanLu]

GET_BUILD

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

## API Changes

unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any', access: 'private' }
unknown top level export { type: 'any', access: 'private' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'identifier', name: 'Column' }
unknown top level export { type: 'identifier', name: 'Column' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }

[LFDanLu]

@alexasselin008 looks like a flakey ci build, rerunning

EDIT: holding off the merge for this release, will go in right after though!