feat: add count, countWhere, streamCount, and streamCountWhere

[SupposedlySam]

[Project Card Link]()

Reason for Change

Adds:

• count
• countWhere
• streamCount
• streamCountWhere

Proposed Changes

Use v4 of cloud_firestore to expose access to the new count function for count and countWhere. Provide access to a continually updated count through snapshots.

Type of Change

• [x] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

Checklist

• [ ] Readme Updated
• [ ] Example project updated
• [x] Tests added/updated
• [x] Changelog updated
• [x] Pubspec version updated