Add nested hugo:vars support

[bep]

Relates to #14699.

@import "hugo:vars";
@import "hugo:vars/mobile" (max-width: 650px);

And then:

{{ with resources.Get "sass/main.css" }}
  {{ $opts := dict
    "vars" (dict 
         "primary-color" "blue"
          "mobile" (dict "somemobilevar" "foo)
     ))
  }}
  {{ with . | css.Build $opts }}
      <link rel="stylesheet" href="{{ .RelPermalink }}">
  {{ end }}
{{ end }}

• The above should be trivial to implement for css.Build
• I'm not sure about css.Sass (or: it would be trivial to implement, but I'm not sure Dart Sass' import supports the relevant syntax) .

[sYanXO]

Is this issue still open? @bep
if yes would like to work on it. :)

[bep]

@sYanXO any issue marked with either Proposal or Triage is not ready to be worked on. We first need to decide it this is something we want to do. We just added the vars support and need to test it out some more before we add complexity.