github: set enhancement too

There is no obvious difference after creating issue between
bug report and feature request at a glance.

When handling initial waiting-for-triage label, "enhancement" label
should be also reconsidered.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -1,6 +1,6 @@
 name: Feature request
 description: Suggest an idea for this project
-labels: "waiting-for-triage"
+labels: ["waiting-for-triage", "enhancement"]
 body:
   - type: textarea
     id: description