Merge #6472

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 <[email protected]>

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