Feature request: Arbitrary JS Operation

It would be nice to be able to create custom operations without having to build a copy of CyberChef, especially for specific one-off use cases. I imagine this could be done by creating an operation, taking JS as an argument, and `eval`ing it when the operation is ran.

The issue identifies no files, tests, or entry points to inspect. First clarify the custom-operation interface, execution constraints, and security expectations around evaluating JavaScript; define done as a documented, tested way to run a one-off operation without rebuilding CyberChef.

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