Description of the new feature/enhancement

I was trying the new settings UI and I found that adding a new color scheme is actually harder than using the json editor. When using the json editor I can directly copy the colors. But with settings UI I cannot do that anymore, which frustrates me a bit. I'd like to propose the feature to import a json file as color scheme. This way all those color schemes in https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/windowsterminal can be used easily.

This is inspired by iTerm2 on macOS.

Proposed technical implementation details (optional)

Ideally it should be just adding a button, parse the json & save.