Add Windows Terminal scheme definitions

[amn]

This adds one JSON file for each variant of the color scheme (black, dark, light and white) for use with the Windows Terminal (https://apps.microsoft.com/detail/9n0dx20hk701) application.

No installation is currently provided, unfortunately. A Powershell script I wrote in hopes of automating splicing in contents of the added files into to the application's settings file, quickly ran into JSON serialization issues present with at least the Windows' default Powershell version, so I had to cut my losses there. For now, this will have to do. One is expected to splice the JSON content from the added files into the settings file, using one's own method(s).