Add new issue template for a general feature

This is meant for changes, that should be discussed first,
but neither fit into the bug nor in the api-proposal template/category.

.github/ISSUE_TEMPLATE/03_general_feature.yml

@@ -0,0 +1,26 @@
+name: 💡 General feature (not including API proposals)
+description: Add a new feature that does not change the public API surface
+title: "[Feature]: "
+labels: [feature]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        You have a general feature request? Cool, thanks for that.
+  - type: textarea
+    id: background
+    attributes:
+      label: Background and motivation
+      description: Please describe the purpose of the new feature.
+      placeholder: Purpose
+    validations:
+      required: true
+  - type: textarea
+    id: alternative-concerns
+    attributes:
+      label: Alternative Concerns
+      description: |
+        Please provide alternatives you have thought about. 
+      placeholder: Alternative designs
+    validations:
+      required: false