Allow more specific theme modifications with semantic highlighting enabled

Original context - https://github.com/dotnet/csharp-tmLanguage/pull/266#issuecomment-1663042472

With textmate, it is possible to have very specific configurations for highlighting, for example you can configure it to highlight the 'event' keyword differently from the 'class' keyword because it has separate scopes for each (though both under the keyword scope).

With semantic highlighting, these all end up under the 'plainKeyword' scope so you cannot differentiate.

This tracks the request to have semantic highlighting be able to differentiate between these, potentially with modifiers.