Don't allow `<nothing>` to suppress unreachable errors

```py
def foo(data: list[str]):
if data is None:
print("hi") # error, unreachable
if data is None:
raise Exception # no error
```

Due to the unsoundness of Pythons runtime, mypy allows `raise` and other `` forms to suppress "unreachable" errors to allow for a smooth way to validate types and data dynamically.

I don't like this as you could:
- run into situations where you unintentionally create a `` on an unreachable line
- have dead code lying around that is no longer needed.

I don't know what the best way to handle this is though, as there would be no way to detect if the code in question is dead or intentional.

### Additional ramblings
Ideally there should be a special error type for this exact purpose, with hard and soft checks depending of if it's syntactically or type-based unreachable:
```py
class UnreachableAssertion(AssertionError):
"""Used to indicate when a branch should be unreachable."""
pass
```
```py
assert False
message = "among"
assert_unreachable(message) # type-based unreachable, no error
raise UnreachableAssertion(message) # syntax based unreachable, no error
raise UnreachableAssertion(message) # error, absolutely unreachable
```
```py
assert True
raise UnreachableAssertion(message) # error, statement is reachable
```

Or maybe that is over-engineering a problem that doesn't exist, maybe just only special casing `assert_never` is enough.

The issue names no files or tests; start by locating the unreachable-error handling and the existing treatment of `raise`, `assert_never`, and other `` forms. Done should be a settled rule for these cases, with regression coverage for the examples in the issue.

Written by the indexing model from the issue text.