Replace markdown issue templates with issue forms (pydata#6119)

Author: andersy005

.github/ISSUE_TEMPLATE/bug-report.md

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+name: Bug Report
+description: File a bug report to help us improve
+title: '[Bug]: '
+labels: [bug, 'needs triage']
+assignees: []
+body:
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: What happened?
+      description: |
+        Thanks for reporting a bug! Please describe what you were trying to get done.
+        Tell us what happened, what went wrong.
+    validations:
+      required: true
+
+  - type: textarea
+    id: what-did-you-expect-to-happen
+    attributes:
+      label: What did you expect to happen?
+      description: |
+        Describe what you expected to happen.
+    validations:
+      required: false
+
+  - type: textarea
+    id: sample-code
+    attributes:
+      label: Minimal Complete Verifiable Example
+      description: |
+        Minimal, self-contained copy-pastable example that generates the issue if possible. Please be concise with code posted. See guidelines below on how to provide a good bug report:
+
+        - [Minimal Complete Verifiable Examples](https://stackoverflow.com/help/mcve)
+        - [Craft Minimal Bug Reports](http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports)
+
+        Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
+        This will be automatically formatted into code, so no need for markdown backticks.
+      render: python
+
+  - type: textarea
+    id: log-output
+    attributes:
+      label: Relevant log output
+      description: Please copy and paste any relevant output. This will be automatically formatted into code, so no need for markdown backticks.
+      render: python
+
+  - type: textarea
+    id: extra
+    attributes:
+      label: Anything else we need to know?
+      description: |
+        Please describe any other information you want to share.
+
+  - type: textarea
+    id: show-versions
+    attributes:
+     label: Environment
+     description: |
+       Paste the output of `xr.show_versions()` here
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/bugreport.yml

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+name: Feature Request
+description: Suggest an idea for xarray
+title: '[FEATURE]: '
+labels: [enhancement]
+assignees: []
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Is your feature request related to a problem?
+      description: |
+        Please do a quick search of existing issues to make sure that this has not been asked before.
+        Please provide a clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+    validations:
+      required: true
+  - type: textarea
+    id: solution
+    attributes:
+      label: Describe the solution you'd like
+      description: |
+        A clear and concise description of what you want to happen.
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Describe alternatives you've considered
+      description: |
+        A clear and concise description of any alternative solutions or features you've considered.
+    validations:
+      required: false
+  - type: textarea
+    id: additional-context
+    attributes:
+      label: Additional context
+      description: |
+        Add any other context about the feature request here.
+    validations:
+      required: false