## Summary of the Pull Request
Introduces `IInheritable` as an interface that helps move cascading settings into the Terminal Settings Model. `GlobalAppSettings` and `Profile` both are now `IInheritable`. `CascadiaSettings` was updated to `CreateChild()` for globals and each profile when we are loading the JSON data.
IInheritable does most of the heavy lifting. It introduces a two new macros and the interface. The macros help implement the fallback functionality for nullable and non-nullable settings.
## References
#7876 - Spec Addendum
#6904 - TSM Spec
#1564 - Settings UI
#7876 - `Copy()` needs to be updated to include _parent
b603929214