Warn against iterating strings

```py
def f(s: str):
for item in a:
...
```

Iterating is most often a mistake. We should have an option to prevent these usages .

```py
a: Iterable[str] = "asdf" # warn: erm, that's silly
```

The issue provides no file or test entry point. Start by locating the checker logic and option definitions for iterable and string diagnostics, then clarify whether both loop iteration and assigning a string to Iterable[str] should be covered. Done means the option consistently handles the shown usages and has tests for them.

Written by the indexing model from the issue text.