6472: Add `static` modifier for associated functions r=matklad a=p3achyjr
Adds static semantic token modifier to associated functions, resolves#6194
## Info
- Associated functions are more-or-less equivalent to static methods in other languages. This PR checks, for each function, whether that function has a self_param, and whether it's enclosed in a trait/impl.
## Changes
- Added method ```is_associated``` to code_model::Function. This basically gets the source from the ast, and checks whether the enclosing scope is an impl or trait.
- Added `static` to HighlightModifiers
- Added unit test
## Tests
- Ran ```cargo test```
Co-authored-by: Anatol Liu <axlui@anatols-mbp.lan>
0a715cfbd2