fix: assists no longer break indentation
Fixes https://github.com/rust-lang/rust-analyzer/issues/14674
These are _ad hoc_ patches for a number of assists that can produce incorrectly indented code, namely:
- generate_derive
- add_missing_impl_members
- add_missing_default_members
Some general solution is required in future, as the same problem arises in many other assists, e.g.
- replace_derive_with...
- generate_default_from_enum...
- generate_default_from_new
- generate_delegate_methods
(the list is incomplete)