Interpolation within text fields in workflow nodes.

[ChausseBenjamin]

Goes along with caido/caido/issues/1452

Note

I'm not sure if we want to talk about the engine first and then the syntax or
the other way around. We should discuss before merging.

Syntax first advantage: If users already know about how the engine works they see how to make use of it straight away.

Engine first advantage: Some tings like the custom print functions and the tip about pre-computing complex interpolated values in a separate code-block make more sense if you have context about the engine.

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