Create copilot-instructions.md (#47204)

Author: Forgind

.github/copilot-instructions.md

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+Code should match the style of the file it's in.
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+Changes should be minimal to resolve a problem in a clean way.
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+User-visible changes to behavior should be considered carefully before committing. They should always be flagged.
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+Large changes should always include test changes.
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+When considering how output should look, solicit advice from baronfel.