New rule: find unused placeholders for f-strings

[Sanchoyzer]

ruff==0.1.14

def func(my_var: int) -> int:
    var = my_var * 2
    print('var is {var}')
    return var

It might be useful if Ruff will find such places, I made a typo: 'var is {var}' instead of f'var is {var}'

[trag1c]

I think this is a duplicate of #8151

[charliermarsh]

Agreed -- I'd like to add this!