Semantic Highlighting support

[bigyihsuan]

Semantic highlighting has less support compared to other languages. Now, this may be low-priority for pretty colors, but it makes things more cohesive (and for me, the pretty colors help differentiate between variables).

Some cursory inspections show that there's no semantic token types associated with the various tokens:

[lnicola]

You should give https://marketplace.visualstudio.com/items?itemName=matklad.rust-analyzer a try, see https://blog.rust-lang.org/2022/02/21/rust-analyzer-joins-rust-org.html.