"disabledProfileSources" is saved to `CascadiaSettings` _not_
`GlobalAppSettings` (and, even then, it's only read when it's used,
never saved). This PR specifically detects if it was defined in
settings.json, and copies it over when the settings are serialized.
## Validation Steps Performed
1. Added "disabledProfileSources" to settings.json, then serialized. -->
"disabledProfileSources" is now maintained.
2. Updated `CascadiaSettings` serialization test
Closes#9032
## Summary of the Pull Request
This adds `ToJson` functions to `Profile`, `GlobalAppSettings`, and `ColorScheme`. They are used in `CascadiaSettings` to completely serialize an instance of the settings model. Thanks to #7923, all of the settings are `std::optional`, and `JsonUtils` only writes out values that are actually populated.
`CascadiaSettings::WriteSettingsToDisk` serializes the current settings and writes them to the settings.json. A backup file is created with your old contents.
#### Limitations:
- all of the color schemes are serialized regardless of them coming from defaults.json or settings.json
- keybindings/actions are copied/pasted
## References
#1564 - Settings UI
TSM Specs (#6904 and #7876)
## PR Checklist
* [x] Tests added/passed
