[expo-cli] rename eas build commands & restructure files

[dsokal]

I made the following changes in this PR:

• Renamed expo build to expo eas:build (to avoid confusion with expo build:*).
• Renamed expo build-status to expo eas:build:status (to be consistent with our naming conventions and avoid conflict with expo build:status).
• Setting EXPO_ENABLE_NEW_TURTLE is not required to use EAS Builds anymore. The new way of using the new build service is by creating eas.json. If the eas.json file doesn't exist in the project root directory the new commands won't be registered in expo-cli.
• I renamed the directory with new build commands from build-native to eas-build.
• I cleaned up (restructured) the files in eas-build so the new files structure makes more sense now.
• Few other minor changes like renaming types, updating the build command's description, etc.