Allow the 16 basic ANSI colors in addition to HEX colors

Prerequisites

[x] I have read and understand the CONTRIBUTING guide
[x] I looked for duplicate issues before submitting this one

Description

First of all I totally love this project and it makes using the CLI much more enjoyable for me.

Ok now to the actual idea:

I use oh-my-posh for the powershell which I mainly run inside the windows terminal. Basically there are a couple of nice themes for the terminal, which map the 16 basic ANSI colors to nicer-looking ones.

So I thought about how I could re-use the color scheme from the windows terminal theme in my oh-my-posh theme without copying all the colors over and fine-tuning them there.

The old oh-my-posh used the 16 basic colors which were adapted by the windows terminal theme quite nicely.

In the end i would like to be able to specify the basic 16 ANSI colors in a theme .json like in this example segment:

        {
          "type": "git",
          "style": "powerline",
          "powerline_symbol": "\uE0B0",
          "foreground": "black",
          "background": "lightYellow"
        },

Maybe also using a prefix for color names such as "$black" and "$lightYellow" to make the names more recognizable and easier distinguish between hex and name.

Environment

Theme: agnoster / basically any theme
Operating System: windows
Shell: powershell
Terminal: windows terminal

Other

I would like to contribute the code myself.

JanDeDobbeleer / oh-my-posh