Add Cadl.Reflection.Model/Scalar #1655

Implementation issue for #1536

• Add Cadl.Reflection.Model and Cadl.Reflection.Scalar models
• Add to ReflectionNameToKind
• Add tests
  • Include coverage for distinction between object and Model discovered during design review: union of models is assignable to object but not Model
• Update our own decorator signatures to use Model instead of object