[less] identifier expected when using @media (@var) syntax

[romainbraun]

• VSCode Version: Version 1.19.0-insider
• OS Version: 10.11.6

Steps to Reproduce:

1. Declare a less breakpoint variable like @m: ~'min-width: 750px';
2. Use it in a less file with
   @media (@m) { // rules }

Get a [less] identifier expected error on the @m part.