Update issue templates

Author: xsduan

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Anything that goes wrong, or usability stuff we missed
+title: "(bug) "
+labels: bug
+assignees: ''
+
+---
+
+## Description
+
+**What exactly is going wrong? Please be as precise as possible, but you don't have to be too detailed if the bug is easily reproducible.**
+
+## Environment
+- **Unity version (e.g. 2019.3f11):**
+- **Any settings that you think may be applicable:**
+- **OS version if you know this is an OS specific issue. Include as much detail as you need (e.g. Windows 10 Home Edition, macOS 10.15 Catalina):**
+
+## Steps to Reproduce
+- **Do you remember what activity you were in when the issue happened?**
+- **What steps did you take?**
+- **What happens when you take those steps?**
+- **Can you reproduce it consistently, or does it only happen sometimes? If it is inconsistent, do you recognize any patterns of when it occurs? (you don't have to be the expert though -- casual observations will suffice)**
+- **Are there any other known errors that aren't related to this one, but might look the same?**
+
+## Expected Behavior
+**If it isn't immediately obvious, what do you expect to happen? Is it clearly an error, or does it work as intended, but behave in such a way that makes the component awkward or complicated to used?**

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: "(feature)"
+labels: ''
+assignees: ''
+
+---
+
+## Description
+
+**What exactly would you like to see? Be precise, but you don't have to work out the details here.**
+
+## Use Cases
+
+**If it isn't super obvious why someone would want this feature, please explain where you would use it. If there is more than one use case, then please list them here.**
+
+**Would any of these changes conflict with other features? If so, how would you propose to resolve them?**
+
+## Mockups
+
+**Do you have any visualizations, or flow charts? They would be appreciated, but not strictly required.**