## Summary of the Pull Request
This fixes a bug where renaming/deleting a color scheme would not update profiles that referenced it.
This also adds detection for renaming a color scheme to a name that is already in use, and adds appropriate UI for that.
## References
#6800 - Settings UI Epic
## PR Checklist
* [X] Closes#8756
## Detailed Description of the Pull Request / Additional comments
`Model::CascadiaSettings` was updated to have a `UpdateColorSchemeReferences()` function that updates all profiles referencing the newly renamed color scheme.
`Editor::ColorSchemesPageNavigationState` now takes and exposes a `Model::CascadiaSettings`.
When a color scheme is renamed or deleted, we use `CascadiaSettings` to update our list of color schemes appropriately, then call `UpdateColorSchemeReferences()` to update the profiles.
The tricky part is that `Profile` does not store a direct reference to `ColorScheme`, but rather the name of the color scheme. See [this tread](https://github.com/microsoft/terminal/issues/8756#issuecomment-760375027) for a discussion on this topic.
## Validation Steps Performed
Repro steps from #8756 when renaming/deleting a referenced color scheme.
## Demo
9700598ecb
