Improve suggestions for unconstrained parameters in `impl` blocks

When an unconstrained type or lifetime parameter is detected in an
`impl`, provide more specific help based on its usage:
- If the parameter is entirely unused, suggest removing it.
- If it is used in the `impl` body but not the `Self` type, suggest
  including it in the `Self` type and the struct definition.

This also adds a comprehensive UI test for these cases.