Operation request: Dean Edwards base62 unpacker

### Summary
A new operation that allows for decoding of eval based javascript packing.
I've been attempting to incorporate this operation however I know zero JavaScript. Both examples below are reliant on the eval() operation which is throwing errors during "npm run lint".

### Example
Open Source Examples:
[p_a_c_k_e_r_unpacker.txt](https://github.com/gchq/CyberChef/files/2622191/p_a_c_k_e_r_unpacker.txt)
http://dean.edwards.name/unpacker/

My attempt:
[FromBase62.txt](https://github.com/gchq/CyberChef/files/2622185/FromBase62.txt)

Errors:
35:9 error Unexpected var, use let or const instead no-var
36:9 error Unexpected var, use let or const instead no-var
38:13 error eval can be harmful no-eval
39:13 error eval can be harmful no-eval