Create use-case.md

Author: BoxyUwU

.github/ISSUE_TEMPLATE/use-case.md

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+---
+name: Use case
+about: Describe your experiences using this feature
+title: '[Use Case]: '
+labels: A-generic-exprs
+assignees: ''
+
+---
+
+### Code description
+
+Describe what you were trying to do with `feature(generic_const_exprs)`. Links to source code are always appreciated!
+
+### What worked well
+
+Describe what aspects of `feature(generic_const_exprs)` worked well for you.
+
+### What worked less well
+
+Describe what aspects of `feature(generic_const_exprs)` did not work well for you. Perhaps they were confusing, or you weren't able to get your code to compile the way you wanted? Or perhaps it was just not ergonomic.