Implement RUF027: `Missing F-String Syntax` lint (#9728)
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## Summary
Fixes #8151
This PR implements a new rule, `RUF027`.
## What it does
Checks for strings that contain f-string syntax but are not f-strings.
### Why is this bad?
An f-string missing an `f` at the beginning won't format anything, and
instead treat the interpolation syntax as literal.
### Example
```python
name = "Sarah"
dayofweek = "Tuesday"
msg = "Hello {name}! It is {dayofweek} today!"
```
It should instead be:
```python
name = "Sarah"
dayofweek = "Tuesday"
msg = f"Hello {name}! It is {dayofweek} today!"
```
## Heuristics
Since there are many possible string literals which contain syntax
similar to f-strings yet are not intended to be,
this lint will disqualify any literal that satisfies any of the
following conditions:
1. The string literal is a standalone expression. For example, a
docstring.
2. The literal is part of a function call with keyword arguments that
match at least one variable (for example: `format("Message: {value}",
value = "Hello World")`)
3. The literal (or a parent expression of the literal) has a direct
method call on it (for example: `"{value}".format(...)`)
4. The string has no `{...}` expression sections, or uses invalid
f-string syntax.
5. The string references variables that are not in scope, or it doesn't
capture variables at all.
6. Any format specifiers in the potential f-string are invalid.
## Test Plan
I created a new test file, `RUF027.py`, which is both an example of what
the lint should catch and a way to test edge cases that may trigger
false positives.
