feat(github): Add pull request and issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a bug or incorrect documentation
+title: 'BUG: '
+labels: 'C-bug'
+assignees: ''
+
+---
+
+<!--
+  Thank you for your report! Please describe your problem here. For questions,
+  use the Typst community discord: https://discord.gg/2uDybryKPe. Feel free to
+  remove any of the sections below if they don't seem useful.
+-->
+
+## Description
+
+## Steps to Reproduce the Problem
+1.
+1.
+1.
+
+## Expected Behavior
+
+## Actual Behavior
+
+## Context
+- Platform:
+- Version:

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Request a feature or change in behavior
+title: 'FR: '
+labels: 'C-feature'
+assignees: ''
+
+---
+
+<!--
+  Thank you for your feature request! Please describe your request here. For
+  questions, use the Typst community discord: https://discord.gg/2uDybryKPe.
+-->
+
+## Description
+A clear and concise description of what the problem is.
+
+## Proposed Solution
+A clear and concise description of what you want to happen.
+
+## Alternative Solutions
+A clear and concise description of any alternative solutions or features you've considered.
+
+## Additional Context
+Add any other context or screenshots about the feature request here.

.github/PULL_REQUEST_TEMPLATE.md

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+See commit messages.