Feature request: Improve highlighting by using sourcemaps

### Summary

Operation highlighting is currently calculated using offsets. For example, [highlighting "Hello" in the input will highlight the corresponding bytes in the output](https://gchq.github.io/CyberChef/#recipe=To_Base64('A-Za-z0-9%2B/%3D')To_Hexdump(16,false,false)&input=SGVsbG8sIHdvcmxk).

This is not particularly reliable when formats are changed or for more complex operations. A better solution would be to generate some form of sourcemap while running certain operations. This would reveal exactly how the input maps to the output.

Some research is required to work out how to carry this out efficiently and how best to architect the solution.