Add an issue template

Author: marijnh

.github/ISSUE_TEMPLATE/issue.yaml

@@ -0,0 +1,25 @@
+name: Issue Report
+body:
+  - type: markdown
+    attributes:
+      value: Thanks for reporting an issue. If you have a question or request for support, use the [forum](https://discuss.codemirror.net/), **not** the bug tracker.
+  - type: textarea
+    id: descriptin
+    attributes:
+      label: Describe the issue
+    validations:
+      required: true
+  - type: input
+    id: browser
+    attributes:
+      label: Browser and platform
+      description: If there is any chance at all that this is browser or platform related, please let us know which ones you tested. If IME or a virtual keyboard was involved, please mention which.
+    validations:
+      required: false
+  - type: input
+    id: try
+    attributes:
+      label: Reproduction link
+      description: When practical, it helps a lot if you provide a script that reproduces the issue in [the CodeMirror sandbox](https://codemirror.net/try/) or another online code environment.
+    validations:
+      required: false