Create design_proposal.md (#2366)

.github/ISSUE_TEMPLATE/design_proposal.md

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+---
+name: 🤔 Design proposal
+about: Collaborate on a design for a feature/fix or other change
+labels: design-proposal
+---
+
+<!--
+This template is useful to build consensus about whether work should be done, and if so, the high-level shape of how it should be approached. Use this before fixating on a particular implementation.
+-->
+
+## Summary
+
+1-2 sentences. Say what this is about.
+
+## Motivation and goals
+
+1-2 paragraphs, or a bullet-pointed list. What existing pain points does this solve? What evidence shows it's valuable to solve this?
+
+## In scope
+
+A list of major scenarios, perhaps in priority order.
+
+## Out of scope
+
+Scenarios you explicitly want to exclude.
+
+## Risks / unknowns
+
+How might developers misinterpret/misuse this? How might implementing it restrict us from other enhancements in the future? Also list any perf/security/correctness concerns.
+
+## Examples
+
+Give brief examples of possible developer experiences (e.g., code they would write).
+
+Don't be deeply concerned with how it would be implemented yet. Your examples could even be from other technology stacks.
+
+<!--
+# Detailed design
+
+It's often best not to fill this out until you get basic consensus about the above. When you do, consider adding an implementation proposal with the following headings:
+
+Detailed design
+Drawbacks
+Considered alternatives
+Open questions
+References
+
+If there's one clear design you have consensus on, you could do that directly in a PR.
+-->