9825: Generate default impl when converting #[derive(Default)] to manual impl r=Veykril a=yoshuawuyts
Similar to https://github.com/rust-analyzer/rust-analyzer/pull/9814, but for `#[derive(Default)]`. Thanks!
## Follow-up steps
I've added the tests inside `handlers/replace_derive_with_manual_impl.rs` again, but I'm planning a follow-up PR to extract these to `utils/` so we can share them between assists - and maybe even add another assist just for the purpose of testing these impls (e.g. `generate_default_trait_body`).
The step after _that_ is likely to fill out the remaining traits, so we can make it so whenever RA auto-completes a trait which also can be derived, we provide a default function body.
Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>