Format nested string quotes

[TomFryers]

Is your feature request related to a problem? Please describe.

Currently, Black (even with --preview) won’t do anything to the following code

f'N is {things["N"]}'
f"N is {things['N']}"

Describe the solution you'd like

It would be nice to be able to format these two expressions in the same way.

Describe alternatives you've considered

The obvious question is which one to choose, but I think it’s clear that the latter is more natural considering Black’s current style.

Additional context

• Perhaps this should be under --preview.
• Triple-quoted strings have a similar problem, but they are much rarer.

[JelleZijlstra]

Duplicate of #567