Fix ROT13 false behavior

[ItisCaleb]

ROT13 will produce incorrect result when rotating numbers with negative amount.

Example

At current version, rotating A0123 with rotate amount -2 will produce the result Y4567

But the correct result is Y8901

[a3957273]

Nice catch, guess we never thought about putting negative numbers into the ROT13 converter for numbers! Would you be willing to add a new test for this operation to test negative numbers?

[a3957273]

Given numAmount is only used in this case, it might be easier to define it locally in this section, e.g:

                } else if (rotNumbers && chr >= 48 && chr <= 57) { // Numbers
                    const numAmount = 10 - (Math.abs(numAmount) % 10);
                    chr = (chr - 48 + numAmount) % 10;
                    output[i] = chr + 48;
                }