Test CSS variables

[weinand]

Test for #3077:

• Windows - @bpasero
• OS X - @aeschli
• Linux - @Tyriar

Use case: Support intellisense, go to & peek definition, find all references, and decoration for CSS variables in CSS, SASS & LESS files.

In a CSS file (.css)

• Declare a CSS variable either in root selector rule ( eg :root { --css-var: red; } ) or in any local rule.
• Assign this variable to a property in any of the rules (say .class1 {background-color: var(--css-var);}
• Test if the intellisense is working when you want to use assign a variable to a property in a rule. Intellisense shows all declared rules in the complete file.
• Test intellisense being inside var() function and check for variable suggestions
• Test go to & peek definition on a variable where it is assigned to a property.
• Test find all references on a variable is showing all references to that variable.
• Test the decoration (color) of the variable
• Test above cases with multiple variables with some declared and some not.

In a SASS file (.scss) and LESS file (.less)

• Test intellisense, go to and peek definition and find all references of CSS variables.