Give struct fields `mutable` semantic token modifier

[lunacookies]

Could rust-analyzer emit the mutable semantic token modifier for struct fields when their type is a mutable reference, or when self is mutable or is &mut Self?

[Veykril]

Since we do the same for locals if their type is a mut ref this sounds good to be consistent. For when the struct instantiation itself(be it a local or whatever) is mutable.

For fields when the owning binding is mutable does sound like an interesting thought as well, though there we probably want to be careful in that if we have a foo.bar.baz, where foo is a mutable thing and bar is a &T we don't want baz to be highlit as mutable.

Makes me also wonder if we should have a second mutable modifier that is used for actual mutable accesses unlike the current one that is for this is mutable, but might be used immutably 🤔

[lunacookies]

Makes me also wonder if we should have a second mutable modifier that is used for actual mutable accesses unlike the current one that is for this is mutable, but might be used immutably 🤔

Ooh that is a good idea; would that be use for &mut methods and assignments?

[Veykril]

Ye receiver of mutable method calls and the place for when writing to places(assignments) was my first thought here.