throw error on invalid `url` func

[alexander-akait]

.function-inside {
  background: url(func('./img.png'));
  background: url(func)'./img.png'));
}

[alexander-akait]

/cc @TrySound What do you think about this?

[TrySound]

Is there a definition of invalid?

[alexander-akait]

Yes, should we throw an error, or use greedy parsing (do not stop parsing on first ))