Add issue templates (#1254)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: A reproducible problem
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+Good bug reports include a failing test! Writing a test helps you to isolate and describe the problem, and it helps us to fix it fast. Bug reports without a failing test or reproduction steps are likely to be closed.
+
+Here’s an example test to get you started.
+https://gist.github.com/ZacSweers/0ef2c5f0e3f61f82063dd8c67d75879f

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Question?
+    url: https://stackoverflow.com/questions/tagged/moshi
+    about: Please ask usage questions on Stack Overflow with the `moshi` tag.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,12 @@
+---
+name: Feature request
+about: Suggest an idea
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+Start by telling us what problem you’re trying to solve. Often a solution already exists!
+
+Don’t send pull requests to implement new features without first getting our support. Sometimes we leave features out on purpose to keep the project small.