Add code action to do string interpolation

[giacomocavalieri]

This PR fixes #4260
And this is what it looks like in practice:

[lpil]

Thank you! One small note inline

[lpil]

Please call it "Interpolate string" in both cases! String means what string.split does, interpolate means to put something in the middle of text, which is what this does.

[giacomocavalieri]

Oh ok! I'm fixing that (and also fixing a small bug I just noticed, might take a couple of minutes 😁)

[giacomocavalieri]

Done! Now it also works properly with pipelines, while before it would generate invalid code

[lpil]

Thank you!!