Add an export option to insert @generated header

[q0w]

https://generated.at/
Now I am doing this with sed like sed -i '1 i\# This file is @generated by Nickel. Do not edit.' "$target" after exporting to yaml/json format

[vkleen]

I think that would work fine for YAML, but JSON doesn't support comments by default. It probably depends on the particular parser implementation whether that works or not.

[q0w]

Oh, yes. I use JSON with comments for GHA workflows/actions

[ad-si]

Could also just be a special field like:

{
  "_⛔️_": "This file is generated by config.ncl. DO NOT EDIT IT MANUALLY!",
  …
}