9734: semantic highlighting: add reference hlmod r=matklad a=jhgg
This PR adds the "reference" highlight modifier!
I basically went around and looked for `HlMod::Mutable` to find the callsites to add a reference. I think these all make sense!
Co-authored-by: Jake Heinz <jh@discordapp.com>
Co-authored-by: Jake <jh@discordapp.com>
1b02cafa43