Closes #5713

Adds a new opt-in rule prefer_scaled_to_fit that suggests using
scaledToFit() or scaledToFill() instead of
aspectRatio(contentMode:) when the content mode is a constant
(.fit or .fill).

As noted in Apple's documentation, scaledToFit() is equivalent to
aspectRatio(nil, contentMode: .fit), so the shorter form is
preferred when no custom aspect ratio is provided.