[css] CSS variables should offer same support as SASS/LESS variables

[dstorey]

With the following code, it should offer to peak/go-to definition when on the var(), the same as for variables in LESS and SASS files:

:root {
    --bg-color: #ccc;
}

body {
  background-color: var(--bg-color);
}

Currently VS Code is not aware of CSS variables.

[dstorey]

Bonus points if it resolves the variable and shows a colour swatch when used in place of a colour :)

[gertcuykens]

Just to make sure we are talking about the same, CSS variables would enable support for

https://drafts.csswg.org/css-variables
https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables
https://www.polymer-project.org/1.0/docs/devguide/styling.html

[sandy081]

Closing this as the changes are merged into the master