Rename theme file to enable theme development features

[arcticicestudio]

Extracted from #143 submitted by @svipas

An undocumented feature for theme extension developers is to ensure the name of the JSON file is suffixed with -color-theme.json. This enables the JSON scheme validation for the theme API allowing developers to validate the implemented theme keys, showing warnings about deprecated keys and providing full auto completion, field documentations and color previews (color picker) for the HEX format.
This change aligns Nord to the official bundled and default themes by adapting to the naming scheme without being a breaking change since the theme is identified by it's extension ID as well as the _metadata field in the package.json and not by the name of the theme file (which would be odd since a theme can provide multiple theme files).