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 <axlui@anatols-mbp.lan>

crates/ide/src/syntax_highlighting.rs

@@ -6,7 +6,7 @@ pub(crate) mod tags;
 #[cfg(test)]
 mod tests;
 
-use hir::{Local, Name, Semantics, VariantDef};
+use hir::{AsAssocItem, Local, Name, Semantics, VariantDef};
 use ide_db::{
     defs::{Definition, NameClass, NameRefClass},
     RootDatabase,
@@ -746,6 +746,9 @@ fn highlight_def(db: &RootDatabase, def: Definition) -> Highlight {
                 if func.is_unsafe(db) {
                     h |= HighlightModifier::Unsafe;
                 }
+                if func.as_assoc_item(db).is_some() && func.self_param(db).is_none() {
+                    h |= HighlightModifier::Static;
+                }
                 return h;
             }
             hir::ModuleDef::Adt(hir::Adt::Struct(_)) => HighlightTag::Struct,