Diagnostics.Append: can now take varargs & skips nil diags

[azr]

👋🏼 Thought this would be a nice — non-breaking change — to have. Let me know what you think 🙂 .

Note that there is that comment Note that this modifies the array underlying the diagnostics slice which I'm pretty sure is not true. To my understanding, this would return a new slice.

[hashicorp-cla]

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