HIP: `warn` template function

[phil9909]

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Summary: This HIP suggest adding a new warn template function.

In addition to that, it gives some ideas how Helm (or plugins) could make use of these warnings besides just dumping them to stderr. This could be future improvements that don't necessarily need to be part of the HIP, but I added them anyhow to show the vision in have for this feature.

It's in a pretty rough state and I'm looking for early feedback. Also, contact me, if you interested in co-authoring this HIP.