Add label to feature_request.yml

It seems like a good idea to have the label 'Feature' to be
automatically added to an issue with that form used.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -1,6 +1,7 @@
 name: Feature request
 description: Suggest a feature
 title: "[Feature] <title>"
+labels: ["Feature"]
 body:
 - type: checkboxes
   attributes: