meta: add initial issue templates

This commit adds the following issue categories:
- Bug report
- Feature request
- Support

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'Type: bug'
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Enable plugin '...'
+2. Run command '...', _or_ try to complete command '...', _etc._
+3. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots or recordings**
+If applicable, add screenshots or record an [asciinema session](https://asciinema.org/) to help explain your problem.
+
+**Desktop (please complete the following information):**
+ - OS: [e.g. macOS]
+ - Zsh version [e.g. 5.6]
+ - Terminal emulator [e.g. iTerm2]
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for Oh My Zsh
+title: ''
+labels: 'Type: feature request'
+assignees: ''
+
+---
+
+**Is your feature request related to a particular plugin or theme? If so, specify it.**
+The name of the plugin or theme you'd like us to improve.
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+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/ISSUE_TEMPLATE/support.md

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+---
+name: Support
+about: Request support for any problem you're having with Oh My Zsh
+title: ''
+labels: 'Type: support'
+assignees: ''
+
+---
+
+<!--
+1. Try to make sure the issue is due to Oh My Zsh
+2. Include as much information as possible
+-->