Flag: using "whether" in a sentence with "or not"

`whether` implies the alternative, so writing this is invalid

| Invalid | Valid |
|---------|------|
| I wonder whether the water is cold or not| I wonder whether the water is cold|
| | I wonder if the water is cold or not|

No files or tests are named in the issue. Start by locating the existing grammar-rule implementations and their tests, then add coverage for the invalid and valid sentence forms shown; done means the invalid form is flagged while both valid alternatives remain accepted.

Written by the indexing model from the issue text.