Support less syntax

[FoundTheWOUT]

@import 'xxx';
.body{
  &-content{}
}

1. The less compiler is load from user workspace, therefore I'm not sure how to write the test for this feature.
2. The less compiler will erase the class that owning no value, this would cause some unexpected behaviors at some time. For example, for the example code above, we will get nothing in the Completion Inteligent.

before:

after: