node-sass is deprecated

[Mikilll94]

What problem does this feature solve?

node-sass is officially deprecated.
https://github.com/sass/node-sass

vue-cli should inform the user about that fact. Dart Sass should be the suggested SASS implementation.

What does the proposed API look like?

When selecting a CSS preprocessor the prompt should inform the user that node-sass is deprecated.

Pick a CSS pre-processor (PostCSS, Autoprefixer and CSS Modules are supported 
by default): (Use arrow keys)
❯ Sass/SCSS (with dart-sass) 
  Sass/SCSS (with node-sass) <!-- THIS IS DEPRECATED NOW -->
  Less 
  Stylus 

[fangbinwei]

Close via #6090