Add issue templates

Author: Greg Hendershott

Diff for: .github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug Report
+about: Report a problem with existing features
+labels: bug
+---
+
+To help me help you, please:
+
+1. Open any racket-mode or racket-repl-mode buffer.
+
+2. <kbd>M-x</kbd> `racket-bug-report` <kbd>ENTER</kbd>.
+
+3. Copy and paste that, here.

Diff for: .github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: Feature Request
+about: Suggest an idea for a new feature
+labels: enhancement
+---

Diff for: .github/ISSUE_TEMPLATE/question.md

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+---
+name: Question
+about: Ask a question about how to use racket-mode
+labels: question
+---