### TL;DR
Refactored `SpecProvider` singleton access pattern and reorganized adapter/extension code structure.
### What changed?
- Changed `SpecProvider.getInstance()` to `SpecProvider._` for cleaner singleton access
- Moved adapter/extension code from `_common` directory to dedicated `adapters` and `extensions` folders
- Consolidated adapter extensions into a single file
- Removed unused dependencies from package.json
- Deleted unnecessary schema files
- Extracted `MobileSpecsPatches` class into the mobile patching code
- Updated all references to use the new `SpecProvider._` accessor
### How to test?
- Verify all specs are properly registered and accessible via `SpecProvider._`
- Test adapter functionality for HTML, Markdown, Notion HTML and plain text
- Check mobile-specific features and patches work correctly
- Ensure preview functionality works in both page and edgeless modes
### Why make this change?
- Improves code organization by properly separating adapters and extensions
- Simplifies singleton access pattern
- Removes unnecessary dependencies and files
- Makes the codebase more maintainable by consolidating related functionality
