Update feature_request.md

Added prompt for whether the API already exists in other webviews.

Author: champnic

.github/ISSUE_TEMPLATE/feature_request.md

@@ -16,6 +16,7 @@ assignees: ''
 <!--
     A clear and concise description of what you want to happen. This may include:
     - Description of what API need to be added or changed.
+    - Does this API already exist in another WebView-like product? (CEF, WebBrowser, etc.)
     - Code that shows the surface area of the API.
     - Code that shows real world scenarios, and how they would otherwise be handled.
     - Details showing the usage/consumption of the proposed new API, and alternatives (e.g. not having this API).