Support extending settings

[cameri]

Pledge
I will be implementing this myself.

Is your feature request related to a problem? Please describe.
It seems likely that we will end up with multiple settings.json files in the codebase. For maintainability, to reduce duplication and increase re-use, settings files should allow for a extends field that can point to another settings file, which may contain default values.

Describe the solution you'd like
Allow extends field at the root of any settings.json to point to another file with defaults.

Describe alternatives you've considered
None

Additional context
None