4903: Add highlighting support for doc comments r=matklad a=Nashenas88
The language server protocol includes a semantic modifier for documentation. This change exports that modifier for doc comments so users can choose to highlight them differently compared to regular comments.
Example:
<img width="375" alt="Screen Shot 2020-06-16 at 10 34 14 AM" src="https://user-images.githubusercontent.com/1673130/84788271-f6599580-afbc-11ea-96e5-7a0215da620b.png">
CC @woody77
Co-authored-by: Paul Daniel Faria <Nashenas88@users.noreply.github.com>