Add Leaflet.heat support / Rename composables to fit the standards

[Gugustinette]

🔗 Linked issue

Resolves https://github.com/nuxt-modules/leaflet/issues/16

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [x] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• useLHeat composable was added to support Leaflet.heat
  • Related tests and documentation were added
• useMarkerCluster was renamed to useLMarkerCluster to fit the standards

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