Add issue/PR templates (#242)

Author: ryantrem

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**For questions, please use the [forum](https://forum.babylonjs.com)**.
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Other**
+ - Platform: [e.g. Win32, UWP, HoloLens, Android, iOS]

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**For questions, please use the [forum](https://forum.babylonjs.com)**.
+
+**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/pull_request_template.md

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+**Describe the change**
+
+A clear and concise description of what the change is.
+
+If this change addresses any issues, be sure to [link this PR to those issues](https://docs.github.com/en/issues/tracking-your-work-with-issues/creating-issues/linking-a-pull-request-to-an-issue).
+
+**Screenshots**
+
+If applicable, add screenshots to help explain your change. Consider using tools like *Giphy Capture* or *ScreenToGif* to capture animated gifs. If you are using physical devices rather than emulators/simulators, consider using tools like *Vysor* or *scrcpy* to project your device's display to your dev machine to allow for screen capture.
+
+**Documentation**
+
+Have you updated the [documentation](../README.md) to reflect your changes? (*Yes* or *N/A*)
+
+**Testing**
+
+Have you [tested](../README.md#testing-in-the-playground-app) the final iteration of your changes? (*Yes* or *N/A* or *I need help testing on [Android|iOS|Windows]*)