Move the issue template

Having a singular issue template named `.github/ISSUE_TEMPLATE.md` is using the legacy workflow from GitHub.
GitHub nowadays recommends for issue templates to be placed in a `ISSUE_TEMPLATE` directory and use front matter, to allow for multiple descriptive issue templates.

Author: jrfnl

.github/ISSUE_TEMPLATE.md renamed to .github/ISSUE_TEMPLATE/bug_report.md

@@ -1,3 +1,9 @@
+---
+name: Bug report
+about: Create a report to help us improve
+
+---
+
 ## Problem/Motivation
 
 > (Why the issue was filed)