Syntax highlighting for types #89

Types don't seem to have a colour in the highlighting. I've tried every recommended colour theme and they all have the same problem. This screenshot uses Dark+.

I would prefer types such as string, option, bool and the t from Js.Null.t to be given a colour. The lack of any highlighting in option<bool> seems particularly weird. It would also be nice if the type definition (attributeSelector) had a colour.

I'm using v1.0.6 with vscode freshly updated to 1.54.1.