Special highlighting for variable props

[Monkatraz]

Was part of my old PR. Quoting myself:

In detail, normal properties use the entity.other.attribute-name.svelte scope, while CSS variables use the support.type.property-name.svelte scope. These tend to have slightly different syntax highlighting in most themes, but the way they are highlighted is consistent with CSS properties. I figure that it should be clear - even in highlighting - that these are not really attributes in the normal sense.

Monokai Dimmed shows this well (My theme, Dark+, and Light+ all highlight those two scopes the same way)