Can't autocomplete css-next vars and mixins

[weichensw]

CSS vars and mixins typically start with -- which are not considered symbols by SymbolProvider

.class {
  background-color: var(--custom-color);
  @apply(--custom-mixin);
}

ref: https://github.com/atom/autocomplete-plus/blob/master/lib/symbol-provider.coffee#L52

[weichensw]

Some initial investigation shows it's a combination of SymbolProvider doesn't consider word begins with -- is a "word" and autocomplete-css set excludeLowerPriority: true

atom/autocomplete-css#56