Low-priority feature request: narrow using dynamic `locals()` function

### Describe the Bug

I would like to request the feature that pyrefly understands "if "variable" in locals()" and does not give unbound-name errors in such cases (see below):

```python
def fun(arg):
if arg==True:
variable=1
if "variable" in locals():
print(variable) # gives unbound-name error: `variable` may be uninitialized
```

### Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSImMYABGAK7oAUqATgOYCUiAOujQJoRabdgF4xAFVb0YvfoMUA3NhFTZYYgIx9FwmjxArWajTEND%2BUXAGNUUOE267Fg4ifQAXJsdOxONADENOwQSvA0jHiMmAC06KgAtjA0MKysuKyINAAGvuqwOTSJqKQ02CmMEOgQnmpQEABeMJggADQg9HUOJOSIIMEAqt21ZQzoNnW46HB8fFS0YJklngD66PSJFaxM%2BNnVngGxAHw0cJ5ZLgKsMJ70rPxghgBym9vZwPgAvoZ87SBkG5gKCkQieXCJKAUYIABVIQJBZwwOAINBs00g7HuqCm6EIfGCAGUYCkABaeTzEOCIAD0NMB1BBhEy7BpMHQNMwtjgNPR6Ex2NxNLomRoqBU0AKKT5AtYOIg0xouGIuLg%2BPQZE8pOmsXCrDgCv4YgMIAAzIQtAAmX7oEBfDqoSZhGAAMWgMAoaCweCIZDtQA

### (Only applicable for extension issues) IDE Information

_No response_