new rule - fstring syntax in non-fstring

```py
asdf = 1
"value is {asdf}" # the user probably meant to make this an f string
```

to avoid false positives, it should only complain when the expression inside the `{}` refers to an existing variable. that way, most usages of the `format` function won't trigger false positives:
```py
x = "{foo}" # no error because `foo` variable does not exist
x.format(foo="bar")
```