Support the semantic token API

[reteps]

What is the problem this feature will solve?

This would be needed for other language servers to highlight documents (see rcjsuen/dockerfile-language-server#273)

@sndst00m I believe that we'd need to implement semantic highlighting (see guide) to achieve what you want. I can't personally dedicate the time to implement this at the moment but I'm happy to review PRs ☺️

Originally posted by @mads-hartmann in #303

What is the feature you are proposing to solve the problem?

Expose the needed APIs for semantic highlighting.

What alternatives have you considered?

N/A