Add support for gradient colours

[schmorrison]

Adds a struct which implements TerminalColor interface. Added steps to Render for gradient application to text.

[muesli]

This looks pretty cool, even though it does complicate the common code paths a bit. I guess we could try and separate a bit of the nested rendering code.

[schmorrison]

Thanks. TBH I totally forgot about this one. I remember that I couldn't figure out a clean entrypoint to accomplish this. It works and passes tests......... but I'm not sure if you have any plans to modify the Render method ever again 👹