Is it possible and, if yes, does a solidity editor exist that estimates gas per line of code while typing? I haven't found anything like that as yet. Either it is difficult or simply nobody has bothered to build such a tool yet.
I don't think it exists for solidity because you can write solidity over multiple lines and its not an issue. The gas fees come once its compiled. remix has gas estimates for each function and the actual gas fee of each operation is outlined in the yellow paper: http://gavwood.com/paper.pdf.