refactor: Extract SpecificationExtensions type and apply to all spec interfaces
- Created SpecificationExtensions interface in 3.0.x and 3.1.x spec.d.ts
- Applied to all 27 interfaces that support extensions in each version
- Replaced inline `[extension: x-${string}]: unknown` with extends clause
- Also added SpecificationExtensions to JsonSchemaDraft2020_12
Co-authored-by: mrlubos <12529395+mrlubos@users.noreply.github.com>