add GitHub issue templates

Author: greenrobot

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something is broken in ObjectBox? Help us to fix it.
+title: ''
+labels: 'bug'
+assignees: ''
+
+---
+
+:rotating_light: First, please check:
+ - existing issues,
+ - Docs https://swift.objectbox.io/
+ - FAQ page https://swift.objectbox.io/faq
+
+**Describe the bug**
+A clear and concise description in English of what the bug is.
+
+**Basic info (please complete the following information):**
+ - ObjectBox version (are you using the latest version?): [e.g. 1.3.1]
+ - Reproducibility: [e.g. occurred once only | occasionally without visible pattern | always]
+ - Device: [e.g. iPhone 11]
+ - OS: [e.g. iOS 13]
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Put '...'
+2. Make changes to '....'
+3. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Code**
+If applicable, add code to help explain your problem.
+ - Include affected entity classes.
+ - Please remove any unnecessary or confidential parts.
+ - At best, link to or attach a project with a failing test.
+
+**Logs, stack traces**
+If applicable, add relevant logs, or a stack trace.
+
+**Additional context**
+Add any other context about the problem here.
+ - Is there anything special about your code?
+ - May transactions or multi-threading play a role?
+ - Did you find any workarounds to prevent the issue?

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Question
+    url: https://stackoverflow.com/questions/tagged/objectbox
+    about: Ask how to do something with ObjectBox, or why something "isn't working" (Stack Overflow).

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Something is missing in ObjectBox? Suggest your idea.
+title: ''
+labels: 'enhancement'
+assignees: ''
+
+---
+
+:rotating_light: First, please check:
+ - existing issues,
+ - Docs https://swift.objectbox.io/
+ - FAQ page https://swift.objectbox.io/faq
+
+Start with a clear and concise description of what problem you are trying to solve.
+Often there is already a solution! 
+
+**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 (e.g. platform or language) about the feature request here.