[tern.mmrm] Add new argument `residuals` to `g_mmrm_diagnostic()`

Idea:
- [ ] allows to choose whether one wants 'raw' or 'standardized' residuals when using `type = "fit-residual"`
- [ ] this would be ignored when `type = "q-q-residual"` since then only 'standardized' residuals make sense
- [ ] refactoring of current function code might be valuable given length and complexity, although interface should stay (`g_mmrm_diagnostic()` for users)

Note: This came in from a user as a feature request.

Start by reading the existing g_mmrm_diagnostic() implementation and tracing how it handles type = "fit-residual" and type = "q-q-residual". Define the requested residuals choices and verify that the new behavior is ignored for Q-Q residuals; done means both residual modes work without changing the existing user-facing function interface.

Written by the indexing model from the issue text.