chore: add WDL-specific instructions for copilot

[adthrasher]

Adding an initial set of instructions for Copilot specific to WDL and our internal best practices.

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• You have added a few sentences describing the PR here.
• The code passes all CI tests without any errors or warnings.
• You have added tests (when appropriate).
• You have added an entry in any relevant CHANGELOGs (when appropriate).
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[adthrasher]

I didn't write anything for code review, but if we're interested in letting copilot review code, we could add further instructions.

[a-frantz]

I've stopped using copilot (or any LLM for that matter) in my local development. Is this for local instances of copilot or is this for GitHub instances for code reviews and the like?

[adthrasher]

Right now, all versions of copilot should use the file. There is a excludeAgent keyword that goes in the header block that can be used to exclude certain agents. I didn't write any code review-specific instructions, but we could add those. I still have copilot turned on and I've used the agent mode a couple of times through github.com.

[a-frantz]

🤔 for local development, I don't think we want to commit this to the repo. But I'm game to test out GitHub's AI for code reviews. I don't have high hopes or grand expectations, but as it's only ever the two of us reviewing code in this repo another "opinion" might be worth adding to the mix.

So I guess we can give it a spin! We can always revert if it proves unhelpful (which truthfully is what I'm expecting 🤣 )