Auto merge of #13041 - DorianListens:dscheidt/gen-fn-self-assoc-2, r=Veykril

Auto merge of #13041 - DorianListens:dscheidt/gen-fn-self-assoc-2, r=Veykril

feat: Generate static method using Self::assoc() syntax

This change improves the `generate_function` assist to support generating static methods/associated functions using the `Self::assoc()` syntax. Previously, one could generate a static method, but only when specifying the type name directly (like `Foo::assoc()`). After this change, `Self` is supported as well as the type name.

Fixes #13012