Add css.ChromaStyles #15112

We have a CLI command for this, with Chroma's recent addition of dark/light modes, it would be convenient to generate these on-the-fly. This would allow this to be configured in hugo.toml and also allow the user an easier way to find the correct settings.

E.g.

{{ $dark := css.ChromaStyles dict 
  "targetPath" "css/croma_dark.css"
   "style" "monokai"
   "modeSelector" "dark"
    "mode" "dark"
}}
<link rel="stylesheet" href="{{ $dark.RelPermalink }}">

This would be even more useful if we fix #15103.

• The options would be targetPath + the options in https://gohugo.io/commands/hugo_gen_chromastyles/#hugo-gen-chromastyles
• Some sharing of code between the gen command in the commands package would be sensible.
• Creating the Resource should be delegated to create.Client.FromString.